I'm baby sitting 4 little shit heads as we speak and they're all playing this shit. What the hell is it? Looks like an extended lego game.
fairly good comparison actually.
its an automatic generated world. everything is made of blocks (like lego!). As the player, you can build stuff, mine to the bottom of the world etc.
then there is crafting - you put different stuff together to create other stuff. for example a couple of sticks and some stones can make you basic tools like pickaxes, axes, shovels etc, allowing you to gather and make better stuff.
when you get to quite advanced levels you can set up redstone circuits - the equivalent to electicity, to create automatic farms, railways, mob traps and so forth. not to mention you can make a portal to another dimension known as "the nether".
basically it is like a huge sandbox game, you can do pretty much what the hell you want. we have a MP server up which makes it all the better.