Hopefully you saw this before starting the game. Warlords' Battlecry requires you to build a hero who has a class and a nation and then, be it skirmish or campaign you get to play as one of the 16 nations. My purpose is to help you get started and point what you get with each nations. I tried all nations with a 5 Charisma hero (any below and it's additional cost. Any above and it's a discount). Note: You can leave at start the general part, since they are tier 5 units and you may not use them that much anyway.

Knight: One of the three human nations, they have pretty cheap infantry and three types of cavalry, which are the main power source of the nation. They also have a fairly decent cheap anti-flying unit named Dancing Sword which not only you can spam (due to cheapness and relatively short build time) but it can spawn from your dead units (Dancing Swords excluded obviously). They also have a good flying unit in the form of archon and a variety of upgrades that helps in pretty much everything.
Their general (tier 5: can act as a secondary hero) is good in melee, can summon Dancing Swords and is good against evil units.

Dwarf: Their units are fairly durable and strong, however they are also quite slow to build and pretty slow-moving. They also got a ballista but despite being a pretty powerful ranged option, they take quite a while to build and I don't know whether they are worth it. This nation's main power is in melee and their ranged options aren't that many.
Their general is really powerful in melee but unfortunately doesn't have any spells.

Undead: Skeletons are possibly the cheapest source of infantry and in a unique twist they can be instantly (provided you have the right buildings) upgraded to other units with either ranger or melee capabilities with the flaw that if you want to get the best from them you need a lot of resources (upgrading 10 of them to the last melee level requires more than the resource cap). They have cavalry but I'm not entirely sure they are worth their money. Most of their upgrades benefit the skeletons' upgraded forms.
Their general is the Vampire doing a bad Count Dracula impression, drains life and can summon bats.

Barbarian: The second human nation but also one of the three barbarian nations. All of their units are fairly cheap and fast-moving. While their tier 1 and 2 units have a lot to be desired, they later gain access to cavalry, which isn't that tanky but it's fast and not that expensive. They also have a fairly cheap "bombardment" flying unit. The Barbarians can work as a horde faction due to the fact that they gain a building solely with population upgrades.
Their general has a good attack and is pretty cheap wile also being relatively fast to build, making him the only spammable general. He doesn't have any spells (none of the barbarian factions' generals do), but receives a bonus against large creatures.

Minotaur: Another barbarian nation. Their infantry is a bit costly, but it is also one of the most powerful, with the ability to heal by killing sheep (which they can also buy). They also have a semi-decent ranged unit that gives you resources per kill. Their spellcaster has decent spells and they have an assassin that is a bit easy to kill for my taste. Their upgrades focus on pressing on their combat advantage.
Their general is a bit costly requiring a fair amount of all resources and takes a while to recruit but is nearly omnipotent, since one of them can kill dozens enemies or nearly destroy the enemy camp on easier difficulties. They are also gigantic, being able to engage in melee with flying units.

Orc: I was surprised with the amount of variety orcs had. From the goblin building you can use zerg rush tactics thanks to the goblins' short recruit time, cheapness and pretty fast movement, but also thanks to the fact that the other units in the building can also summon goblins one way or another (the goblin ballista randomly through attacks and the goblin chieftain through his skill). Their other units are fairly durable with decent attack. The orc unit also gains bonuses that help them deal with archers fairly easily.
Their general is fairly powerful and can hit multiple units if they are close. They are also giants.

High Elf: Their infantry leaves a lot to be desired. However, they get two pretty good cavalry units in the form of Dragon Knights who receive a bonus against Dragons and the Unicorns who can heal your units. They also have two support spellcasters with decent attack (low health though. Keep them far from enemies). They also have a decent flyer in the form of wyverns and their scorpion is a rather good siege weapon with a multi-target attack. All their units are pretty fast allowing for hit-and-run tactics. Their upgrades allow units to further excel in their roles, even having 3 upgrades for the Dragon Knights, but they also have upgrades that boost healing.
Their general is the first (if you take them by the row displayed here which is the same with the game) archer general. He is rather fast, but not that strong or durable. He also has a multi-target attack.
Note: High Elves, Wood Elves and Dark Elves have a special research that allows you to combine 4 wisps (their builder) into one Ancient Wisp. This new unit is a bit stronger, but is also helpful due to generating 1 Crystal per second each (crystal is one of the 4 resources)

Wood Elf: These elves have infantry, 2 types of cavalry, one of which is a cavalry archer, 2 spellcasters, treants and 2 additional units that can be used as scouts. All of their units except the treants have pretty good speed but they are rather mediocre in survivability and just tolerable in strength. Their ranged units can once again be used for hit-and-run tactics. One of the spellcasters has a pretty damaging spell. The treants on the other hand are pretty slow but also tanky and powerful. They also get a buildable mine.
Their general following the main theme of the nation is also pretty fast with reduced conversion time.

Dark Elf: Their pikemen are rather unremarkable, however their archers can poison and they have access to a cavalry that is good against heroes (to the point a single one can kill a hero). They also gain access to two kinds of spiders, who are pretty fast and cheap with poisonous attacks, who can harass your enemies and skeletons (without the fun upgrades of the undead though). They also have a pretty good assassin that also generates gold per kill and two spellcasters that are slightly more durable than their Wood and High Elf cousins (I recommend the spider sorceress). They also gain access to imps (Harass flyer) through a late building and a rather powerful daemon through a random summon ability. They also help if you have an assassin hero since they have a building and uprgades that help with that.
Their general is a melee unit that can go invisible for a while or until he attacks.

Fey: Their units are really weak to the point where pretty much anything can one-shot them, but they are cheap, they have weirdly high attack and really high speed. They also have a cheap unit that helps generate gold (one of the 4 resources). They also have a building that gives levels to your units, increasing their stats and trust me, you gonna need to get as much you can as fast as possible. I think this qualifies as a zerg rush nation.
Their general is also rather weak, but benefits from the levels and this way she can get rather impressive. She can also cause Fear to enemies.

Dark Dwarf: The Dark Dwarf main infantry includes golems, which are really durable, quite powerful, but they are really slow. Additionaly, Dark Dwarves gain access to a lot of siege units, three of which are unique to them; the first is a cheap and fast suicide unit, while the others are a really powerful cannon and an impressive anti-air one with the disadvantage that both are slow to build and move. They also borrow wraiths and shades from the undead, probably to make up for the lack of speed.
Their general is also a golem. It is pretty durable and strong, can collect resources from destroyed buildings, but unfortunately is really slow.

Daemons: The only faction to include a tier 1 flyer, which works pretty well as a harassment unit. The rest of their line-up is impressive with units having balanced stats, while at the same time all stats are pretty high. Only disadvantage is that if you want the better ones you need to upgrade your base building, which requires the most resources out of all nations.
Their general has impressive stats, can summon daemons and even banish enemy summoned creatures.

Empire: The third and last human nation, the Empire features rather average infantry (e.g. their pikemen are about as strong as Knights' squires), but they also include powerful elephants and an instantly-built powerful but expensive cavalry. You can also try your luck with their ability to summon randomly creatures from other factions, where they helpfully give you choice between a random weak, a random strong unit or a random general (it's shared between that type of building, so multiple buildings don't help). Their mercenaries are a bit pricey, but thankfully they improve on that and there is a chance you get something good (during my try I got a Minotaur King).
While other factions usually had one, the Empire has three generals (if you include mercenary generals they have them all): the White Mage that supports and heals your troops, the Red Mage that has a spell to increase fire resistance and one to cause fire damage and the Black Mage, with a damaging spell and the ability to drain enemy mana. All of them are rather weak in durability though, so keep them back if possible.

Ssrathi: Their basic infantry has decent attack and durability and can poison the enemy, while their cavalry may be the fastest in the game even though not that durable. They also have an early game spellcaster with the ability to heal your troops, who also has upgradable range. The most amusing about this nation is their ability to recruit dinosaurs, who despite their slow build time are pretty powerful.
Their general is not impressive stats-wise but she can boost the armor of your troops, so that's nice.

Swarm: Another zerg rush nation (the other being Fey), I would say I prefer them over Fey, due to their troops being more durable and slightly stronger, while being just as fast. They also have two troops that cause poison and access to upgrades that reduce training time.
Their general has a damaging fire spell and the capability to summon a fire elemental.

Plaguelord: Their basic infantry cause disease stopping health regen, despite not having that impressive stats (even though they somewhat improve on that). They have missile units with very good attack and a suicide unit, which isn't as fast as the Dark Dwarf one, but can cause much more damage. They also have hydras, which are fairly durable with splash damage and good attack and bonus: they can be upgraded to two different kinds: Ice and Pyro; besides the obvious different element in attacks, the first is faster to build and slightly faster, while the latter is more durable and has higher attack.
Their general has a decent melee attack, even though his durability doesn't recommend it, but can summon scouts and can cause disease.

-All units move at the slowest unit's pace. Keep that in mind, so that for example you won't mix your fast wood elf cavalry with the slow-moving treants.

-All nations feature flyers and siege units. The flyers and siege units are shared across many of the nations, usually complimenting the nation's playstyle. Both types function fairly similarly though, so I only included flyers and siege units that stand out.

-All nations also have a couple of dragons and a titan. Those are endgame units with destructive capabilities, which unfortunately aren't that different to warrant a seperate mention for each faction.

-Considering this game has a lot of staff, there may be something missing. In that case don't hesitate to mention it. Just be polite about it.