If you're familiar with Minecraft, the fastest explanation is "2-D Minecraft". However, it's not so black and white: The long explanation is it's a 2-D game that features exploration, building, crafting, and fighting off hordes of eldritch abominations. Slimes casually jump around in the daylight, murdering poor bunnies and other critters. At night, the Zombies and flying Demon Eyes come. There is some dark, evil thing that's slowly altering the world, either the Corruption (purple, "evil" biome) or the Crimson (a more gory flesh based evil). You must gear up and prepare, as powerful bosses will eventually come to face you. You must build, explore, mine, and craft to survive, discover new tools and places, and become the biggest badass ever.