Blizzard has setup a collection of cinematic and in-game movies to be available to download. The file itself is roughly 312mb and uses the Blizzard downloader to download. For all you not in the Beta, the downloader is a bit torrent client used by Blizzard to deliver their patches quickly. These movies and the movies are being made available to stress test their delivery method and help them improve it before release.

For full details check out the WoW beta site article here.