What is Closed Captioning?
Published: Jun 21, 2007
Closed captioning (CC) allows hearing-impaired people, people learning English as an additional language, people first learning how to read, people in a noisy environment and others to read a transcript or dialogue of the audio portion of a video, film or other visual presentation. As the video plays, text captions are displayed that transcribe, although not always verbatim, what is said and by whom and indicate other relevant sounds.
The term “closed” in closed captioning means that not all viewers see the captions—only those who decode or activate them. This is distinguished from “open captions,” where the captions are visible to all viewers. Open captions are occasionally referred to as “in-vision” in the UK. Captions that are permanently visible in a video, film, or other medium are called “burned-in” captions.
In the US and Canada, “captions” are distinguished from “subtitles.” In these countries, “subtitles” assume the viewer can hear but cannot understand 
the speech or language, so they only translate dialogue and some onscreen text. “Captions” aim to describe all significant audio content, as well as “non-speech information,” such as the identity of speakers and their manner of speaking.
It has been suggested that the largest audience of closed captioning is hearing people in ESL communities. In the US, the National Captioning Institute noted that ESL learners were the largest group buying decoders in the late 1980s and early 1990s (before built-in decoders became a standard feature of US television sets).
Source: Wikipedia
