Vudu has about 5000 movies currently but the majority are in standard definition. Many studios are beginning to offer HD version however because the release of a movie in HD, while not having to place it on "format war media" Blu-ray and HD DVD can be very advantageous and reach more consumers.
The service works using P2P technology somewhat similar to torrents. "When a user purchases or rents a film from the service, the beginning of the movie is streamed directly from Vudu's servers while the rest is downloaded in the background. Streams are MPEG-4 and converted to HD, so only a 1.2 Mbps connection is required for viewing. Faster connections result in no delay."
Finally, primetime network TV content has gone into beta on Vudu with such hit shows as Arrested Development, Familly Guy, My Name is Earl, and Prison Break being available.
Source:
Betanews