Explained best in symmetry magazine:

"The site allows the user to watch Feynman with subtitles; to take notes that link to specific points in the video timeline and video transcript; and to access expert commentary, bibliographic references and Web links, all also linked to points on the video timeline. The user can search the transcript for keywords and then click on those words to watch that section of the video. However, the multimedia presentation can only be viewed using Microsoft’s Silverlight technology.

Bill Gates says in an introduction on the Project Tuva Web site that he has been hoping to share the Feynman lectures with the world for 20 years.

“I think these Messenger Series lectures [Feynman] gives are the best science lectures I’ve ever seen,” he says.

“I think someone who can make science interesting is magical. And the person who did that better than anybody was Richard Feynman. He took the mystery of science, the importance of science, the strangeness of science, and made it fun and interesting and approachable.”"

Work sited: http://www.symmetrymagazine.org/breaking/2009/07/15/feynman-messenger-lectures-now-available-online/