Audio description is a term used to describe the descriptive narration of key visual elements in a video or multimedia product. This process allows individuals who are blind to access content that is not otherwise accessible simply by listening to the audio. In audio description, narrators typically describe actions, gestures, scene changes, and other visual information. They also describe titles, speaker names, and other text that may appear on the screen.
Audio description is sometimes referred to as video description, descriptive video, descriptive video service, or simply DVS. However, the latter two terms are registered trademarks of WGBH Educational Foundation .
In a video program, audio description is added to the Secondary Audio Program (SAP) channel. In streaming multimedia, audio description can be added by synchronizing the narration track with the visual track. Various media players (e.g., Real, QuickTime, Windows Media Player) have slightly different approaches to supporting this capability. For additional information, see the AccessIT Knowledge Base article What is the difference between SMIL and SAMI? 
Audio description should be carefully scripted and is typically produced by trained professionals. The Media Access Group at WGBH is a leader in audio description and provides extensive additional information on their Descriptive Video Service FAQ . The National Center on Accessible Media (NCAM), another WGBH group, has developed a free software tool called Media Access Generator (MAGpie)  that allows users to add captions and audio descriptions to their multimedia products.
A web search for "audio description" will return many additional providers of audio description services.
-  WGBH Educational Foundation
-  What is the difference between SMIL and SAMI?
-  Descriptive Video Service FAQ
-  Media Access Generator (MAGpie)