High Definition and Closed Captions:
More details at http://help.youtube.com/support/youtube/bin/answer.py?answer=100079
Check out this instructional video on captions and subtitles made by one of your fellow YouTube users, rewboss!
Captions and subtitles are the dialog of a video in written text format. Sometimes they also include information to help folks follow the dialog, like descriptions of music, phones ringing, and other sounds in a video's audio track.