GigaOM explains.

Making legal YouTube mashups just got a whole lot easier. The site’s video editor is now allowing its users to remix existing YouTube videos without violating anyone’s copyright. This is made possible by YouTube adopting Creative Commons licenses, offering users the chance to publish any video under the liberal CC-BY license. It’s a big step forward for YouTube, and a giant leap for Creative Commons, which previously hasn’t played a big role in the web video world.