I forgot to tell you what Minecraft exactly is, so here I go:
First of all it's a game still in Beta, that means it is still in development, and is getting developed mainly by a swedish indie-developer (that means he isn't reliant on a big publisher like EA or Activision). Because he's a indie-developer, he can do with his game whatever he wants, whenever he wants. And that is what Notch does (Notch is the nickname of Minecraft's inventor Markus Persson):
In periods from a few weeks to a few months Notch releases updates which bring short additions and sometimes even huge new contents to the game.
Next time I'll tell you more,
craft 'ya later,