Terraria is a video game developed by Re-Logic. It was first released initially for Windows on May 16, 2011, but has since expanded to several other platforms. The game is a 2D action-adventure, open-world sandbox / platformer in which the player controls a single character in a pre-generated world.
If you're planning to start a new plathrough or play the game for the first time, the wiki is a great place to start by clicking the cover image above.