Let's say that you just bought the game, or have only a few hours, and you want to get better! Well look no further.

Open up the game and skip or don't skip the cutscene. After you watched it, or skiped it, it's time to play! Go and click the box labled Campaign. Then click singleplayer or if it's not there, click "quick match". If you get the option to play Singleplayer, Do. First, choose the campaign. Any will do. Then choose your difficulty. I'd recommend Normal for now. Then pick your character, pick anyone, but when i play singleplayer, i just stick to random. Then the game will choose a character out of the 4 playable survivors for you to play. If you picked Quick Match, the game will ask you if you want to play on a official server or a different server. Just select Official. Then the game will randomly select a random avaiable game. Then, just play the game. Remember. Use your eyes and ears. Listen out for the special infected, that make noises. After you play the game, always say gg.

The 8 survivors (well 7 now) are the main protagonists of this game. Those being the eight lovable characters:
Left 4 Dead 2:

• Coach - A highschool American Football Coach
  
• Nick - A con artist with a shady backstory
  
• Ellis - a young car Mechanic
  
• Rochelle - A news broadcaster

Left 4 Dead 1 (Or simply Left 4 Dead)

• Bill - A old army vet
  
• Francis - A greasy Biker
  
• Louis - A office worker
  
• Zoey - A young horror-movie watcher

The Survivors, gameplay-wise, actually have no distinguishing features tied to them. They all have the same speed, have the same health, and reload and the same speed. The only things that are different are the character models.

Coach
(This is from the Wiki.)
"Coach (real name unknown) is one of the four playable Survivors in Left 4 Dead 2. He was a high-school health teacher and coordinator for the freshman football team before the Infection hit his hometown of Savannah. He is the oldest of the group and takes it upon himself to motivate his fellow Survivors.
Coach has a big heart and a healthy appetite, and a wicked swing with a chainsaw. After a knee injury ended his career as a defensive lineman in college, Coach salvaged his Physical Education degree (barely) and landed a job teaching health at the local high school in his hometown of Savannah.
Working as a defensive coordinator for the freshmen tour team might not have been the best path to a pro coaching career; but it’s come in pretty handy in guiding a group of Survivors to safety. Coach has watched his beloved hometown get ravaged by infected—now it’s time to deliver his own brand of Southern hospitality."

Nick
"Nicolas, or Nick is one of the four playable Survivors in Left 4 Dead 2. He is a gambler with a shady―even violent―past. He seems reluctant to be a part of the group at all and takes the role of "the complainer," finding fault in other people's plans, the increasingly sorry state of his once-fine suit and his teammates' character flaws, as he judges them. This makes him difficult for the others to get along with at first, but that changes as time goes by, as he learns to trust them and proves himself a valuable part of the team.
A lifetime of drifting from city to city, finding back-alley card games and trying to stay out of jail has taught Nick two valuable lessons: Don’t trust anyone, and look out for number one.
He’d come down to Savannah looking for some gullible fish on the riverboat gambling cruise circuit. Instead he found a city about to be engulfed by infection. Now he finds himself forced together with three complete strangers in a fragile alliance that goes against every instinct he lives by. But he’s going to have to learn to trust them if he wants to survive."

Ellis
"Ellis (also known as El) is one of the four playable Survivors in Left 4 Dead 2. He is the youngest of the group. A junior mechanic by trade in the city of Savannah, he presents himself as a goofy, beer-loving country guy with an unsophisticated and sometimes naive world view. Ellis is a mechanic with a love of life, a firm belief in his own immortality, and the ability to treat any setback as a fun dare to impress his friends. Born and raised in Savannah, Ellis divides up his time working at the local garage, hanging out with his buddies, and dropping by for Sunday dinners with Mom—why’d anybody want to live anywhere else?
Then the zombies had to go and spoil it. Now Ellis is looking for new things to occupy his time, and finding plenty. It turns out the zombie apocalypse is one big dare, and there’s no shortage of crazy stuff he can try to impress his new buddies."

Rochelle
"Rochelle (also known as Ro) is one of the four playable Survivors in Left 4 Dead 2. She was employed as an associate producer on a television news station in which role she was sent on assignment to Savannah (the location of the first campaign) to cover The Infection's breaking story. As a low-level associate producer for a big-name news station, Rochelle's job mostly consisted of lugging cables and fetching coffee. But when the outbreak hit and staff started calling in sick, Rochelle got her break: producing a segment from Savannah about the evacuation center located there. She was still setting up the cameras when her big story became a war zone.
But that doesn't mean she's letting go of her dream job yet. Surviving a zombie apocalypse is just something else she can use her wits and drive to produce the hell out of."

The Normal infected, aka The Commons, are the main enemy you will encounter when trying to survive the green flu outbreak. The Commons do different damages. I'll lay them out for you. Keep in mind that they do more damage to the front (where your camera is facing) and less damage to your back, the (opposite of where your camera is facing)
On easy, the commons do 1 damage, to both your front and back.
The default difficulty is normal, where the commons do 2 damage to the front and 1 damage to the back.
The harder difficulty, aka advanced is a good balance between the infected and the survivors. The infected deal more damage overall compared to normal and easy. The Commons do 5 damage to the front and 2 to the back.
The hardest difficulty, aka Expert, is what the title says. Expert. The Commons deal 20 damage to the front, and 10 dmg to the back. Making Expert where most good players play.

How to deal with Common Infected
It's rather simple. Just shoot them. A common infected will usually go down in a few well placed shots. Melee weapons are really effective against Commons too. Just keep in mind that Left 4 Dead 2 hitboxes are wacky. Oh yeah, and utilize your shoves. (or M2). This will perform a shove that will stun a few common infected in front of you. This allows you some breathing room to reload or to swap to your secondary.

But Commons aren't the only zombie types you have to worry about. The biggest roadblocks being the Special Infected.
The Left 4 Dead 1 special infected are:

The Smoker
The Hunter
The Boomer
The Tank
The Witch

I will include the Left 4 Dead 2 special infected in the next section because I ran out of space.

Fun fact! Did you know that until Left 4 Dead 2, There were no female special infected?
All of these bad boys (and girls) are the biggest threat to your well-being, as the commons don't pose a giant threat as long as they don't clump up. Like the Commons, The Specials have direct damage that they do, and they are all unique in their own way.

THE SMOKER
The Smoker is a ranged infected, able to drag a survivor away from the other three. Think of the smoker like a sniper. The Smoker likes high ground, and will not be typically seen wandering around the ground levels. He is actually one of the three original Special Infected. The Smoker has 250 HP. Using his primary attack by pressing M1 (left mouse button) The tongue, is like a ranged sniper rifle, that drags a survivor away. When the survivor is being pulled they could get stuck on zombies, cars, you name it. If the survivor is dragged all the the way to the smoker, the smoker will start to claw at the survivor as the survivor calls out. His secondary attack, is his scratch. using M2 (right mouse button) the smoker will claw. Dealing damage. The Slash doesn't do much, but it will work nicely on a low health survivor. If you see the smoker as a survivor and have a melee weapon, you can attempt to cut the tongue. Wait until the smoker is done with his little taunt/noise then swing. Hopefully you cut the tongue, if you don't try it again. It's hard to master and even do. If you succeed, you'll get an achievement, called Lickety Split.

THE HUNTER
The Hunter is the next special infected and is one of the og special infected, alongside the smoker. In Left 4 Dead, there were a few achievements specifically for the Hunter, like punching him mid-air. The Hunter has a unique trait, when standing and wandering around normally, he makes no sound, when he crouches, preparing to pounce, he will make a racket making it easy to detect the hunter when he's crouched. His Primary attack, the pounce, requires the hunter to crouch and then jump. To perform the attack, use the crouch key, then M1. When you do this, the hunter will leap forward with fast speed, and if you land on a survivor, you will pin them down to the floor, then you start the shred them to pieces. In the case if you are unsuccessful in your kill, and knocked off the survivor, you will be stunned, then you can try to run away. The achievement called: One Hit Wonder can be obtained with the hunter. The achievement requires you to get a 25 Damage Pounce. So how the damage pounces work, are that you need to be in the air for a bit, to get a certain dmg pounce. When trying out this achievement, play on dead center, no mercy, or any map that has a high elevation that you can either climb or jump to. Aim at a survivor, then pounce. Keep in mind that the survivor body is weird, when you see the character model, its actually a second behind the physical model. Try to aim a little further and higher, if you just aim willy nilly, you'll tend to miss short. Then boom, you get urself an achievemento. Now im getting off topic. The Hunters' M2 is the scratch, that has a weird sound effect, when you scratch as the Hunter, its a mixture of the normal scratch sound and door breaking sounds. When the Hunter is a bot, if you shoot at the Hunter, it'll jump away, its like a predator and prey, predators like sneak attacks, when discovered, will lose confidence.

THE BOOMER
The Boomer is the last original special infected from Left 4 Dead. The Boomers' M1 "Attack" is his vomit. When M1 is pressed, the boomer will spit for a few seconds, then will stop. If the boomer is successful in vomiting on a survivor, the stench of the vomit will have the survivor play a voiceline saying that the vomit is gross. Then the Vomit will attract a horde. Try to attack first, or when the survivors are busy/distracted. There is a special property to the boomer. When the Boomer dies, they will explode, coating nearby survivors with bile. The Boomer has the normal scratch attack, nothing super interesting. There is an achievement where you coat somebody in bile without looking at them. This is called Fat Ninja. An easy way to get this achievement is to jump into a massive group of survivors, and start to scratch away.

THE WITCH
The only special infected that is not playable. She will spawn at random points on a campaign, and will patrol along (on the daytime maps) and sit on the ground, crying to herself. (nighttime maps) It is recommended that player-controlled special infected take advantage of a witch. Scratching the witch as a special infected, will agitate her. If a survivor is there when doing so, the witch will pretty much lock onto that survivor.
What maps will have wandering witches or sitting witches?
Witches will patrol on these maps: Dead Center, Hard Rain (Map 1 and 2), The Parish, and Cold Steam.
Witches will be sitting on these maps: The Passing, Dark Carnival, Swamp Fever, Hard Rain (Map 3), No Mercy, Death Toll, Dead Air, Blood Harvest, and The Last Stand.
How to Combat the Witch
The key is to completely avoid the Witch if you can. Do not engage with a witch unless you are confident with your aim. Witches have 1000 HP. While startled, she takes little to NO damage to any attacks except for melee! Burning the witch is a good strategy, but it takes 15 seconds in order to kill her with fire. So finding an area where she cannot reach you is ideal, such as the Saferoom or the ladder inside the barn in Dark Carnival.
If you want to actually kill the witch, the simplest way is dubbed: Crowning. I won't tell you how to do that because there are dozens of guides online on how to do it.
Expert Witches
Witches on Expert is quite the jump. I'm not gonna sugarcoat it. She instantly kills you. No matter what health you're at, she hits you once, you're dead. No bells and whistles, just death.

THE TANK
The Tank. The big boi, thickhead, lots of more names. The most common way of dispatching a tank is to pump him full of lead and throw your mollies on them. While it's the easiest way to dispatch of a tank, He is very dangerous with the help of other infected. While he may look slow, don't be fooled! He actually runs at the same speed as you. Try to use friendly infected to slow down survivors, or snipe them from far away with his boulder throw. While the tank is a big boy, he goes down pretty quick against a coordinated focus fire. Think of the Heavy from Team Fortress 2. High health, but drops quickly to focus fire.
Expert
The Tank on Expert. Wow. What a change. The Tank on Expert basically becomes Tom Brady when it comes to throwing rocks. He leads his throws, so if you keep running in a single direction, you will get hit. The only way to avoid a rock is to finesse him. Oh yeah! Do keep in mind that he will instantly down you with one hit.Whether by Rock or Punch, you are down. If the Tank doesn't feel mercy, he only has to hit you three times while you're downed, and you're dead. Boom. Game over. Keep your distance, but don't be afraid of him.

While it you may look like a big hulk of mass, (which of that you are) you drop quickly to focus fire. Speaking of fire, Fire is the bane of the Tank's life! It will eat away at your health until you die. In order to counter this, Try to bait out Molotovs. Survivors like throwing it at easy places, typically right before you round a corner, or in front of you. Use this to your advantage. In tight corridors, back up while they throw their grenades at you. While you wait for the fire, try to have your special infected teammates (assuming you are playing with actual people) distract the survivors, like have a charger come in and make some chaos, have a boomer come in and puke on everyone. Something, to make it easier for you to approach.

This section was added because I ran out of space.

THE JOCKEY
The Jockey is one of the most inconsistent special infected. One of the new 3 infected, he rides survivors like a horse jockey. Pressing M1 will have the jockey do a lunge, jumping up and attaching to any survivor that's in the Jockey's range. When a Jockey hops onto a survivor, survivors in range will be stunned for a second. Use this to your advantage! When next to high ledges, try to hop on someone, causing the others to fall off the edge. While this may not kill them, this will open up an opportunity for another infected to take the heat off of you. The scratch isn't special. The Jockey is one of the best Infected to distract or drag away a survivor.

THE CHARGER
One of the most famous infected, the charger is both a dangerous, yet hard-to-use. Think about the Jockey or Hunter hitboxes. They suck. However, the charger is the best risk-reward zombie. If paired with a jockey or smoker, he is very dangerous. On many maps, there are high ledges or drop off points. On these points specifically, you can down, or even kill a survivor. Some of the most famous being the hotel in Dead Center, and the rooftops of No Mercy. Boomers can help out chargers by blinding them. Something that is specialized for the Charger, is the extra scratch damage. Instead of a scratch, the charger punches them. It deals more damage, so use pinned players to your advantage. Use them like a punching bag, smack the sh*t outta them!
How to counter them? Try to bait out a charge. The Charger can't change the direction of his charge after used, so use that to your advantage.

THE SPITTER
One of the best support infected. Spitting noxious acid, dealing increasingly more damage, the longer the survivor is in it. M1 will spit out that acid i was talking about. M2 will be your main form of attack. A special property of the spitter is like the boomer, she will drop a puddle of acid upon death. Think of the Medic from Team Fortress 2. Strong with their teammates but weak alone. Be cognizant of where the majority of special infected are spawning. The Spitter is very strong with pinning infected, such as the Smoker, Hunter, Jockey, and Charger. The most preferable special infected partner is the Charger, since it takes survivors longer to get up from the hit than the Smoker or Jockey. Since they recover instantly.

On Expert, Special Infected...get buffed. The change-up is crazy. In general: The Special Infected AI gets a lot more sophisticated.

The Smoker
Smokers on Expert have some nuances that you need to be aware of. Smokers upon spotting a survivor will often immediately attempt to constrict the survivor with their tounge attack. In order to avoid this, look up and scan the rooftops for suspicious green smoke. If you do get grabbed, immediately warn your teammates, as smokers will kill you very quickly!

The Hunter
Hunters on Expert are really unpredictable. Sometimes, they act like a normal hunter. They will crouch and attempt to pounce. However, these vary a lot! Sometimes they will hold their pounce and slowly crawl towards you. Sometimes you will stun a hunter as they reach you, but they will immediately scratch for 40 damage!

The Boomer
Boomers on Expert act differently than on Normal. Expert Boomers will not hesitate to immediately puke on you. To avoid this, attempt to shoot through the wall they're standing behind, or take them out from a distance. If facing a Boomer at close range, always try to shove them first. If you do it quick enough, you can cancel their puke, allowing you to make distance to take them out. Do be aware! Boomers on Expert have a tendency to immediately scratch with their M2, which will deal massive damage!

The Jockey
Jockeys on Expert are the the most unpredictable special infected in the game. There's a reason why the Jockey is highly regarded as the most annoying special infected. Sometimes upon him detecting you, he'll just rush yo ass and hop on your back. Other times, he'll rush yo ass down, but while you're frantically shoving, he'll just scratch yo ass for 20 points of damage. But Jockeys can be rather easy to gun down from a distance, so snipers should prioritize the Jockey if he's even reasonably close.

The Charger
Chargers on Expert are not too different from their normal counterparts. The only thing that changes is their reaction time and aggressiveness. What I mean by that is that they will take less time and won't hesitate to rush yo ass down and punch you for 20 damage.

The Spitter
The Spitter is barely a threat. The only thing you have to look out for is that they are likely to spit immediately once they detect you. Just stay out of the acid and you'll be alright.

The general layout of maps is pretty linear. Most campaigns have 4-5 maps/stages. And while I won't be doing an individual showcase of every campaign, (because that'll take literally hours) Most are simple in their design.

When you play Left 4 Dead 2, The "A.I Director" will dictate how your run will be! The AI director can spawn zombies, items, and even change the layout of maps. When you see zombies, that's the AI director. The only things that aren't dictate, but instead pre-determined are saferoom spawns, crescendo events, and the general layout. The AI director's objective is to make every run different from another! So it'll change different things about the maps. If you and your team is struggling, The AI director may spawn more health kits, or pain pills. And may even remove a tank spawn it was thinking about. Contrast that to when your team is doing well, it may spawn an extra tank, or even put more hordes in.

A survivor's arsenal of weapons is pretty small (well duh, it's not like they have backpacks n' stuff.) But survivors can carry up to 5 items. The two you'll almost always have are the: Primary and Secondary. There are two different types of Primary tiers of weapons. Tier 1, and Tier 2. (well duh.)

Tier 1
While Tier 1 weapons may not be very flashy, it'll do perfectly fine against Commons and the weaker special infected.
The Pump Shotgun - A close range shotgun. High spread, higher damage than the Chrome Shotgun. 8 shells in the magazine. Reloads 1 shell at a time.
The Chrome Shotgun - A close range shotgun. Smaller spread, lower damage than the Pump Shotgun. 8 shells in the mag. Reloads 1 shell at a time.
The Silenced SMG - A close-medium range sub machine gun.. Fair spread and decent damage. 50 bullets in magazine. Reloads entire mag at a fair speed.
The SMG - A close-medium range sub machine gun. Smaller spread but lower damage than the Silenced SMG. 50 bullets in the mag. Reloads mag at the same speed as the Silenced SMG.
The MP5 - A close-medium range sub machine gun. Fair spread, and slightly less damage than the Silenced SMG. 50 bullets in the mag. Reloads mag at a slower speed than the other SMGs.

Tier 2
Now these are the kinds of weapons that you'd think about in a zombie apocalypse! These weapons are the most effective against the Infected.
The Autoshotgun - A close-range fully automatic shotgun. High spread, high damage, more pellets than the Spas-12. 10 shells in the magazine. Reloads 1 shell at a time.
The Spas-12 - A close-range fully automatic shotgun. Medium spread, high damage, less pellets than the Autoshotgun. 10 shells in the magazine. Reloads 1 shell at a time.
The M-16 - A medium-long range Rifle. Small spread, fair damage. 50 bullets in the mag. Reloads entire mag at once. Has the fastest reload of the Rifles.
The AK-47 - A medium range Rifle. Fair spread, the highest damage of the Rifles. 40 bullets in the mag. Reloads entire mag at once. Has a medium reload speed.
The Scar - A medium range Rifle. Small spread, fair damage. 60 bullets in magazine. Reloads mag all at once. Has the slowest reload time of the rifles.
The Hunting Rifle - A long range, single shot sniper rifle. The smallest spread in the game. High damage. Reloads entire mag at once. Has a quick reload speed.
The Military Sniper Rifle - A medium-long range single shot sniper rifle. Small spread, high damage. Reloads entire mag at once. Has a slightly longer reload speed.
(Yes... im not giving these two snipers profiles because they suck.)
The Scout
The AWP

Other kinds/Tier 3
These weapons destroy all infected, but all have drawbacks.
The M-60 - A medium range semi-auto rifle. High damage, fair spread. Has 150 ammo in magazine. Cannot refil ammo!
The Grenade Launcher - A single-shot grenade launcher. Fires grenades that stun all infected except for the Charger and Tank. High damage, no spread. Insanely long reload speed. 1 shot before reload. Reloads entire mag at once. 30 reserve ammo. Be warned, you cannot refil ammo on this weapon! (And no, no rocket jumping. Sorry TF2 fans.)

Secondary Weapons
Originally in Left 4 Dead, there were only pistols as secondaries. While they are fine against Commons on the easier difficulties, they are not the most reliable. With the release of Left 4 Dead 2, Melee weapons and the Desert Eagle, (aka as the Magnum, the Deagle, or big gun.)

Secondaries Available:
Pistol - This weapon that you'll always spawn with at the beginning of a normal campaign. It deals low damage but has a moderate quick firing speed. It has a small bullet deviation, but it's nothing too bad. The Pistol has 15 bullets in a magazine and has infinite ammo. If you go through all of you ammo, empty pistols take more time to reload than if you had some bullets in the magazine.

Dual Pistol - This weapon is the pistol x2. It still has low damage, but has a fast firing speed. It has 30 bullets in a single magazine, since it's just two standard pistols. The reload speed is pretty marginal, but if you go down to one bullet or if you shoot everything in your mag, then the reload speed is painfully slow. It's about the speed of the Scar.

The Deagle - The Magnum. The Desert Eagle. Whatever you want to name it, this is undeniably the most powerful secondary in the entirety of Left 4 Dead 2. It only holds 8 shots in a magazine, but one-shots all common infected, making it completely viable for any map. The only drawback is its low magazine size and inaccuracy when moving.

Melee Weapons
All melee weapons are basically reskins. They always one shot common infected, but some of them have special properties.
Blunt Weapons
Blunt weapons are what the name implies. These weapons bash in skulls.

Baseball Bat - This weapon is a two-handed weapon that swings in an X shape with a crossing hit that comes first. Has a medium swinging speed.

Cricket Bat - This weapon is a two-handed weapon that swings in an X shape with a crossing hit that comes first. Has a medium swinging speed similarly to the Baseball Bat.

Golf Club - This weapon is a two-handed weapon that swings in an horizontal line. It has a fast swing speed and can decapitate zombies to get the FORE! achievement.

Frying Pan - This weapon is a literal frying pan. This weapon is a one-handed. It swings in a horizontal line and has a slow swing speed. Makes a funny sound :)))

Nightstick - This weapon is a one-handed police baton. It swings in a horizontal line despite the animation, and has the fastest swing speed of all of the melees, tied with the Machete.

Shovel - This weapon is a two-handed weapon. It swings in a horizontal pattern, but has the slowest swing speed.

Sharp Weapons
Sharp weapons are what the name implies. These weapons cut things. All of these weapons have the capability to cut Smoker Tounges, resulting in the achievement: Lickety Split.

Fireaxe - This weapon is a two-handed Fire Axe. It swings in a horizontal line. Has a medium swing speed.

Crowbar - This weapon is a one-handed Crowbar. It swings in a horizontal line. Has a slow swing speed. Don't ask me how this is a sharp weapon.
Katana - This weapon is a two-handed Sword. It swings in a slanted X shape. Has a fast swing speed.

Machete - This weapon is a one-handed weapon. It swings in a slanted X shape similarly to the Katana. It has the fastest swing speed, tied with the Nightstick.

Knife - This weapon is a one-handed weapon imported from Counter Strike: Source. It has a very fast swing speed swings in a horizontal line.

Pitchfork - I think this is a sharp weapon??? I don't know, I don't use it. This weapon is a two-handed pitchfork. Has a slow swing speed and swings in a weird pattern. It swings vertically, across in a horizontal line, then vertically like this: \. Like what????

Chainsaw - This weapon is a...two-handed weapon. It doesn't swing in a sense, but instead having multiple small rays shooting outwards from the player. It has a gas meter, which you can only use the chainsaw for 60-ish seconds before it runs out of juice. Once fully consumed, you cannot re-pick it up and it will be removed from your inventory, which is replaced by the Pistol.

Mechanics in Left 4 Dead are somewhat hidden, so here are some of the bigger mechanics in the game.

Death
While playing Left 4 Dead it's really common to y'know. Die. While it may suck, it'll give you an opportunity to relax for a bit. While dying on the easier difficulties is nearly impossible, it's way more common on harder difficulties. Some ways of dying:
Being mobbed, falling off cliffs or edges, being ran over, being squashed, burning alive, drowning, and many more.
Fire Damage
Fire hurts, dude. On easier difficulties, throwing fire on yourself is a good way to get rid of commons, but a melee weapon is so much easier. Throwing fire on yourself or your teammates on Advanced or Expert is NOT, I REPEAT NOT A GOOD IDEA. Throwing fire on your teammates on Easy or Normal may be no biggie, but on Advanced or Expert, it's a one-way-train to becoming a human steak. And a well-done one at that.
Cresendo Events
On many maps, there may be something blocking the way of you going. Like a car, alarmed door, powering up a generator, and many more, but most of them have warning signs.