Terraria is an influential indie 2D sandbox game that's based around exploration, crafting, and fighting spawned enemies. The game features sprite-based graphics and a procedurally-generated world. Its success has led to multiple platform releases as well as had great influence upon later titles, such as Starbound.
Terraria sets you in the world with just a few basic tools with which to explore the world. The rest is up to you. You can mine or chop wood for resources and protect yourself with a sword. Delving deeper into the depths, you'll be able to find better resources and items, as well as encounter a series of monsters that are not friendly, to say the least.
Building a shelter to keep out the creatures is just the beginning, as the game is fully co-op if you want to play with friends, features PvP, and many items to create and find. There are also special rare items and NPCs you can convince to move into your house that grant advantages. The game is also moddable on PC.