Imagine this:
You are washed up on the shore of an unknown continent. Land stretches out in every direction as far as the eye can see, while behind you islands dot the shoreline. Your only goal is to survive... your only tools are your hands and what you build with them.
With your hands you chop down a tree, with the wood from a tree you make a pickaxe and begin carving out the side of a mountain. Not for jollies... for shelter. For when night falls... the monsters come. You find a deposit of coal and fashion torches. You use your ample stone to craft stone tools for yourself as you continue to dig downwards... faint moans and creaks outside from creatures which would love to feast on your flesh.
Day breaks, the monsters burn up in the light... but you are so deep by now you cant tell. You find a deposit of iron and smelt it into steel. Finally, days later, you emerge... clad in shining armor and sword to face the horrors of the night... and win.
But that is just one part of the tale. The next goes on as you take stone harvested from the depths of the earth and raise it up into a mighty stone fortress. Secure in your mastery over your domain... your eye wanders across the untouched waves. You build a boat and set off for unknown shores...
THAT is Minecraft.
The game is in Alpha at the moment. You can play for free, or buy the whole version for $13 like I did. When it goes into beta, which is soon, the price will double. Minecraft's graphics are nothing to drool over, but its blocky style is, in my opinion, quaint and nostalgic. Plus, every world is randomly generated. No two are the same... and the total in game space is larger than the surface of the earth. Its THAT big.
http://www.youtube.com/watch?v=ylVtj-1Ccgg <- brief video. I encourage more youtube searches to see what the game has to offer... its crazy.