Minecraft is a 3D sandbox game, which allows the users to create the content, alone, with friends or team up with other members of the community! Then they can show off their masterpiece structures to everyone.

Minecraft is currently in alpha, meaning it's nowhere near finished, but you're free to try it.
Keep in mind that it's under heavy development and can be a bit unstable during updates.
You can read more on the development blog, or here.

A sneak preview of Survival Mode is now available for people who have bought the game.