Just as Closed Captioning helps the hearing impaired understand films, Audio Description (also known by the WGBH/ PBS-given name Descriptive Video Service) helps the visually impaired. This is aired on what is called a "secondary audio program" channel, or SAP.
What happens is there is a sound track that has a narrator describe the visual action of a video image, taking care to interfere with spoken dialogue and/or song lyrics as little as possible, which means a DVS narrator has just as big a job as a caption transcriber, if not more, to keep out of the action as a neutral observer. However, sometimes the narrator has to take an acting role such as reading out language subtitles of a character in character, ie in Star Wars A New Hope, the narrator effectively acts out Greedo's dialogue in the Cantina scene.
Unfortunately, while some DVD publishers, like Paramount Pictures and Dreamworks Animation, offer described soundtrack settings, the feature so far is nowhere near as common as subtitles or closed captioning, although organizations that assist the visually impaired are doing their best to offer videos with it. There's also the problem that the menus accessing the feature are still entirely visual in presentation, so any seriously visually impaired person will need the help of a sighted person to access the feature.
One of the problems with DVS on television is that because only one additional audio track via SAP was usually offered in the analog age before television was switched to digital, DVS had to compete with foreign language translation tracks (mainly Spanish in the United States, French in Canada), and usually Networks want more viewers rather than provide a public service and preferred translation to description. Many stations don't bother at all with SAP channels so they can avoid complaints from disgruntled viewers who changed to SAP accidentally about how some guy keeps butting in explaining the obvious, why Courteney Cox has a different accent and speaks Spanish, or why a mechanical voice is delivering forecasts to them (most stations with a SAP channel carry a local government weather radio service when DVS or translation isn't needed). This can be annoying when the feature is advertised by a network and isn't there for a viewer in their market.
An attempt by the FCC to mandate DVS in most primetime programming for the largest 25 United States television markets (in effect mandating it nationwide as said markets cover all time zones and networks wouldn't offer non-DVS feeds to other markets based on simple costs) was declared unconstitutional in the courts in 2002, and thus only in the digital age when multiple language tracks can be sent over a television broadcast or be placed on a Blu-Ray has DVS been able to have a footing beyond PBS and Turner Classic Movies, the two biggest supporters of DVS.
However, Congress finally passed a little-known and publicized bill in 2010 to allow the FCC to do the above, and beginning July 1, 2012, a slow-phase in of required description will begin across the country until the top sixty markets have it in 2017.