I don't think I can say it better than the official FAQ:
"The Raspberry Pi is a credit-card sized computer that plugs into your TV and a keyboard. It’s a capable little PC which can be used for many of the things that your desktop PC does, like spreadsheets, word-processing and games. It also plays high-definition video. We want to see it being used by kids all over the world to learn programming."
And some specs, again from the FAQ:
This thingy made a big fuss around it and caught the interest of very many people. The 1st batch of 10000 units was sold in an hour or something.
I got my Raspberry Pi some weeks ago and got some time this days to tinker around it.
Will post soon some tweaks I made and how I got it connected to a Motorola Atrix Lapdock.