What is captioning?
What is subtitling?
Captions make the soundtrack visible to deaf and hard-of-hearing viewers.  Captions are also useful to people who view in noisy environments or with the sound muted.  Besides conveying the actual dialogue, captions describe sound effects, identify speakers or provide other information about the soundtrack.
In North America, closed captions are embedded in analog video as invisible data on line 21 of the vertical blanking interval.  The technical document defining the service is called CEA-608 (formerly EIA-608) and is available from Global Engineering Documents.  Inaugurated in 1981, today this service is included with most broadcasts, cablecasts, and DVDs.  To see the captions, the user activates a CC decoder built into her TV receiver.  Most receivers manufactured since the mid-1990s are equipped with CC decoders.
Better-quality closed captions use the decoder`s "pop-on" mode.  Pop-on captions appear and disappear all at once, for a more pleasing visual effect.  The best-quality captions are crisply timed to the shot changes where appropriate.  Closed-caption decoders also support a "roll-up" mode, which is better-suited to live broadcasts.
Pop-on captioning is delivered as a compact file.  For broadcast, cablecast or home video, a digital video inserter in the tape room reads the timecode track of the master digital recording and inserts the data into the vertical blanking interval.  The resulting video is digitally recorded, creating a closed-caption clone for distribution or duplication.  On DVD and Blu-ray Disc, the caption data are inserted into the video by the consumer playback device so that the user`s built-in CC decoder can display the captions.
The primary CEA-608 captioning service uses data embedded on field 1 (line 21).  The user selects this service by setting her decoder to CC1.  A second CC2 service may also be available on field 1.  Usually, however, if a second closed-captioning service is needed (in Spanish, for example), the captioning data will be encoded on field 2 (line 21) and will be available as the CC3 service.  This arrangement (using both fields of line 21) provides the maximum bandwidth and allows for more crisply timed captions in both services.  Use of  CC1 and CC2 simultaneously works well only with roll-up captioning, with its more relaxed timing expectations.  The same is true of simultaneous CC3 and CC4 services.
The bad news is that TV receiver manufacturers are not required to support CC3/CC4 (field-2) services.  So, before you decide to use CC1 and CC3 for the highest-quality service, you may want to consider how many of your audience will be using TV receivers
whose CC decoders support CC3/CC4.
In Europe, broadcast closed captioning is usually delivered as part of a teletext service.  Captioning and other data are embedded in the vertical blanking interval using more lines and a higher data rate than the older line-21 system used in North America.  In British English, closed captions delivered by teletext are often called "subtitles".
For HDTV receivers, the technical document defining the closed-captioning service is called CEA-708, also available from Global Engineering Documents.  The caption data are encoded into the VANC portion of SMPTE 292M HD/SDI video per SMPTE 334.  During the transition to HDTV, the old-style CEA-608 codes, from the decades-old analog-video line-21 system, are carried forward into the new CEA-708 service as a separate, low-bandwidth, backward-compatible stream.  This stream can be up-converted to the more elaborate 708 codes for HD broadcast, or it can be re-encoded as 608 codes into analog video downconverted from the HD video.  The compatibility stream sustains a constant data rate of 59.94 bytes per second, regardless of the frame rate of the underlying HD video.
Open captions serve the same purpose as closed captions, but they are "burned into" the video and visible to all viewers (no CC decoder needed).  On DVD releases, one of the available subtitle tracks may serve as captions by adding textual sound effects and other audio cues to English subtitles.  Such a service is sometimes referred to as "SDH" (Subtitled for Deaf and Hard-of-hearing viewers).