It's kind of interesting trying to explain Nintendo's 'Wii-U' "platform"... but we'll try because it's just been named on stage by Satoru Iwata.
At heart we've got is what looks like a tablet-like device that can be used as a controller or as a platform to play games... or as a video chat device to use while you watch the telly.
6.2-inch screen
All other controllers can be used with Wii U.
It can be used instead of the TV.
Two analogs
Rumble
Touch screen
Wirelessley transmitted images "without latency issues"
Up to date HD images - without using the screen in your hands.
Or you can play without need for TV.
Or games can be played using two screens.
Video chat while you watch TV
"It was not designed to be a portable video game console" however. It's something new. You use it as a controller, as a screen itself - there are even games in development (Smash Bros. was mentioned) that make use of two screens.
The device can also be used with the DS!
More details as they come in.