During alpha, the Minecraft ran 9.95 (around $13), but that rate increased to 14.95 ($20 or so) upon entering beta. Along with getting the game for five euros less, alpha customers were guaranteed all future versions of the game for free. Buying the game during beta includes all content released until the final game launches sometime this year, which will be followed by another price increase. Bug fixes will remain free for all.
Minecraft, if you're unfamiliar, is extremely difficult to describe. It's a game of exploration, construction, and survival. The world is made of different blocks, each of which is destructible, collectible and reusable in your own creations. Valuable materials are buried deep under the surface along with zombies and other monsters that spawn anywhere it's dark -- and that includes the surface, which has a brief day and night cycle.