you can play single player or multiplayer. plus there are enemies in the world which you must defend yourself from, plus the resources used to construct this stuff must be collected from the game world, which is randomly generated. you can craft weapons / armor / materials from the ores you mine, the wood you collect, and other junk. you can build boats, minecarts and tracks, and other junk. you can build more complicated structures using "redstone" as a sort of simple circuitry. i've only used it to make levers to open locked vaults, but it can be used for more complicated things. if i wasn't at work i'd post some youtube vids that explain it better.
Oh ok, that way sounds better than what I thought. I thought you built stuff that you found in a menu. Having to collect stuff makes it a little more interesting. I might try it out one day.