Closed captioning is a process of subtitling, usually associated with helping people who are deaf or hard of hearing. It is actually material hidden in the video signal (encoded into most TV broadcasts as well as VHS, DVD, LD, etc)... if your TV has the ability (or you have a CC box)-- you can turn it on and get subtitles of what is being spoken.
from the DVD faq
So, it is just text of what is being said, usually provided for the deaf or hard of hearing. http://www.thedigitalbits.com/officialfaq.html#1.45 http://www.robson.org/capfaq/ http://www.ncicap.org/
Just a quick FYI: the difference between Closed Captioning and regular subtitles is that CC also includes descriptive text for sounds besides what is being spoken. Subtitles are writtne with the assumption that the viewer can hear what's going on, so it's just the text, while CC assumes the viewer cannot hear anything.