Gratuitously Stolen from Wikipedia:EIA-708
EIA-708 is the standard for closed captioning for ATSC digital television streams in the United States and Canada. It was developed by the Electronic Industries Alliance.
Unlike most DVB captions, EIA-708 captions are textual like traditional Line 21 captions. However, unlike Line 21 captions, EIA-708 captions include most of the Latin1 character set, and include stubs to support full UTF-32 captions, and downloadable fonts.
EIA-708 Captions are injected into MPEG-2 video streams in the picture user data. The packets are in picture order, and must be rearranged just like picture frames are. This is known as the DTVCC Transport Stream. It is a fixed-bandwidth channel that has 960 bit/s allocated for backward compatible Line 21 captions, and 8640 bit/s allocated for EIA-708 captions. The ATSC A/53 Standard contains the encoding specifics. Either the EIT or the PMT tables contain a caption descriptor which tells you what language the captions are in and tells you if they are regular captions or 3rd grade level captions for language learners.
EIA-708 caption decoders are required in the U.S. by FCC regulation in all 13" (33cm) diagonal or larger DTV televisions. Further, some broadcasters are be required by FCC regulations to caption a percentage of their broadcasts.