It's a sandbox with a little bit of adventure. And Minecraft is made out of block and you can destroy most of these block's. And some with your bare hand's. The game is highly customizable in the way of modding and in the 1.2 update there will be more than 4000 block id's making it easy-er for modders. And that will raise the limit of how many mods you can install in term of content added thing.

Also in 1.2 the level formate is changed to one called anvil. With that the level hight doubled to 256 block. O and one block equals 1 meter. To tripled the building space sins the game is in 3D and not 3D as in vision. And who will know what kind of weird looking building you could expect for this.

And a little more about the game's programming. The game is made in Java and not in C++ or equal. Sins it's made in Java the game depends most on you CPU and not on your GPUSpoiler:

But now more on the game-play. Them game is a sandbox in which you can build by placing blocks and some of the item's can be placed two. And then there are some game-mode's on if Survival the first there was than you got Creative mode and Hardcore. in hardcore the difficulty level is locked in hard and if you die in the game you world is deleted and the rest is like Survival. In Creative you got a infinite amount of resources for building and you can not die in while in this game-mode. And the last one left is Survival. In Survival you can die as many time's as you like. And change the difficulty level to the level you like.
The difficulty levels are Peaceful no monsters that will hurt you, Easy monsters that will hurt you but not that many, normal a lot of monsters that will kill you, Hard the same amount of monster as in Easy but with more health and brain.

So you can see there is a lot of freedom in the sins there is not story line in the game. you can play the game the way you what!