You start in a massive, randomly generated world, and pretty much do whatever you want. The hook is that you can burrow into any surface, as well as build anything out of the materials you mine. There are random mobs running around which drop unique items, a day night cycle, gardening, farming, livestock, crafting... and every week the guy is adding more.
In Multiplayer, you can connect to a persistent server, mine resources, and build structures with other players.
It is being made by just 1 guy, but the most amazing part is that, although the game is still in alpha stage, it is already being sold and he is getting $50,000 every day!! So far, the game has already sold around 100k copies!!
The creator of Minecraft has also gotten the attention of Valve, and apparently he's going to have a meeting with them very soon!!
Here's the stats of the game's website: http://www.minecraft.net/stats.jsp
Statistics
28953 players online, in 609 servers.
489333 registered users, of which 98611 (20.15%) have bought the game.
Here's a video from a newbie to Minecraft and loving the game:
Also, here's a video of what you can build in Minecraft (your imagination is the only limit):
Also, someone built the fucking Earth:
