Its a scaled down version of YouTube that makes it easier to view and play videos. Google hopes this will cause viewers to stay on the Website longer because the videos play continually in the browser.
Leanback is part of YouTube’s effort to move from a video site into a “video operating system” like the experience you have watching TV.
To control Leanback you can use your keyboard directional pad to skip and choose new videos.
Click here to try out Leanback http://www.youtube.com/leanback