Despite its simple graphics and 2D style (a boon for players with old or low-end hardware), Terraria is a wonderfully deep game, having been worked on, updated and added to for a decade after its release, with:

• difficulty options, world sizes, and death penalties to suit everyone, even a peaceful creative mode (I recommend Mediumcore for PATIENT players who want a challenge[thegemsbok.com])
  
• hundreds of varied mobs with pop culture references sprinkled throughout
  
• a day and night cycle, with random events like the Blood Moon or Goblin Invasion
  
• dozens of different bosses with their own movement patterns, attacks, and unique loot
  
• a plethora of immersive insects and animals to collect, sell, or display (including 10 species of butterfly)
  
• activities like golf and fishing, with fun and powerful rewards
  
• tons of pets and minions (combat and non-combat)
  
• weather (including snow and sandstorms)
  
• loads of hairstyles, costumes and armours (and dyes to use on them!)
  
• an advanced building and housing system to attract NPC merchants - keep it simple - or don't! - with myriad decorations, furniture, mechanisms and functions, using wires, sensors, logic gates, liquids, pumps, grates, minecart tracks, timers, teleporters, traps and explosives (try activating a toilet!)
  
• countless treasures, equipment and craftables
  
• holiday events and their own respective mobs, bosses, and NPCs (like Santa at Christmas!)
  
• a recent permanent content crossover with the one and only Don't Starve Together!
• a multitude of things to find and do and build, alone or with up to 254 friends (in theory...)
  
• mod support and a huge modding community!

In short, Terraria is one of my favourite games of all time and easily the best purchase I've made on Steam. Just try it. With Steam's return policy, what've you got to lose?

P.S. Do yourself a favour and avoid public servers and/or experienced players who want to show off or give you loot. There's no XP in Terraria - and inventories can be carried/gifted between servers/characters at will - so without earning your riches, you're only spoiling your sense of progression and ultimately the game's lifespan. Just relax, learn and advance at your own pace. And say goodbye to your friends and family for a while. :)