Closed captions are the text version of the spoken part of a TV show, film, or computer presentation
Closed captioning was developed to aid hearing-impaired people, but it is useful in a broad number of situations. Examples of other usage are environmental, either where the environment tis particularly noisy and the audio cant be heard e.g. a busy public place or where the environment must be quiet and no audio can be played e.g. a hospital ward.
Closed captioning data is encoded within the video signal. The captions can only be seen through the use of a decoder. Decoders are usually built into TV sets, set top box units or software.
In general, an onscreen menu on newer televisions allows you to turn closed captioning on or off. Open captions, in contrast, are an integral part of a transmission that cannot be turned off by the viewer.