From The TV IV
Closed captioning (CC) is a system where text scrolls along the bottom of the television screen during a program, transcribing the aural aspects such as dialogue and music. Closed captioning was intended for the deaf and hard-of-hearing, but is also used by other people such as those learning English as a second language.
The "closed" part refers to the fact that the captioning is encoded into the non-visible portion of the TV signal and is only visible to those with an activated decoder. Since 1993 all television sets 13 inches or larger must have a decoder installed. Some televisions will automatically display closed captioning when the set is muted.
As per the 1996 Telecommunications Act, all programs produced after January 1, 2006 must have closed captioning.