YouTube Links

http://www.youtube.com

The YouTube URL scheme is used to launch the YouTube application or connect to the YouTube website to play the specified video. If your application links to YouTube content, you can use this scheme to play videos from your application.

Application homepage: http://www.apple.com/iphone/iphone-3gs/more-features.html
iTunes link:
HTML Link example: <a href="http://www.youtube.com/watch?v=RRB-m5RGLfY">Play Video</a>
Native application URL string example: http://www.youtube.com/watch?v=RRB-m5RGLfY
Extended example:
Tags:

Unlike some schemes, YouTube URLs do not start with a “youtube” scheme identifier. Instead, they are specified as regular http links but are targeted at the YouTube server.

More information on the Apple Developer website