Terraria is a 2D platform game that tries to take the spirit of Minecraft and apply it to platform gaming - and does a pretty good job of it too.
Like Minecraft, in Terraria you can do pretty much anything you want to do with your tools and imagination. From digging deep underground and finding accessories and money to fighting-off a variety of enemies in numerous "biomes" and gathering wood, stone, ores and other resources, Terraria has an amazing level of depth that Minecraft fans will instantly recognize.
As in Minecraft, your basic aim in Terraria is to create your own world and defend it. You can build a house, a fort, a castle - pretty much anything you want with a little imagination. If you're lucky, people will move in to live in your world and trade with you to help expand your empire. Of course, like Minecraft, graphics are not the strength of Terraria but it's the sheer amount of possibilities and imagination that goes into the game that makes it so enthralling.
If you loved Minecraft, then you should definitely try Terraria.