From the project's website:
What is WatchKnow?
Imagine hundreds of thousands of great short videos, and other media, explaining every topic taught to school kids. Imagine them rated and sorted into a giant Directory, making them simple to find. WatchKnow--as in, "You watch, you know"--is a non-profit online community devoted to this goal.
Kids can create three levels of accounts. All videos are moderated. Videos come from Youtube, Google Videos, Internet Archive, eHow, National Geographic, among others.