Minecraft is a game about placing blocks while running from skeletons. Or something like that... Here, watch this video of me testing rollercoasters in the game instead:
The game is a lot like that, but also has enemies and cave exploring and mining and farming and flowing water and dynamic lighting and a huge (huge) randomly generated world map. AND it has music by C418!
Minecraft is still in alpha, but is rapidly nearing beta. I've only got one more feature to add; multiplayer support. I'm renaming the old version of the game "Minecraft Classic", and will be slowly phasing it out. I won't remove it, though. Just hide it.
The main branch of the game is currently called "Minecraft Alpha", and is under heavy development. It can be a bit unstable at times, so don't get too attached to your save files yet.
You can read more on the development blog, or here.