Audio description is the term used to describe the process of providing descriptive narration of key visual elements in a video or multimedia product. This process allows individuals who are blind to access content that is otherwise accessible only through sight. 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 an art. The words chosen and the placement of those words within the sequence of the video can have a significant impact on the viewer's experience. Therefore, audio description should be carefully scripted and is typically produced by trained professionals. In the United States, no officially sanctioned standard exists that assures quality and consistency across described videos, though a few groups who are active in audio description have documented their recommended practices.
The Media Access Group at WGBH is a leader in audio description and provides some glimpses into their Descriptive Video Service strategies and techniques within their DVS FAQ .
Also, the Audio Description Project, an initiative of the American Council of the Blind, has developed a set of Guidelines for Audio Description , which include recommendations for what to describe, how to describe (both in terms of language and in terms of style and delivery), and preparation.
The most comprehensive set of instructions for audio describers is the Guidance on Standards for Audio Description  developed by the Independent Television Commission (ITC), the body that controls commercial TV broadcasting in the United Kingdom (UK). In the UK, the Broadcasting Act of 1996 requires a minimum proportion of digital television programs to be audio described. Section 20(1) of this act charged the ITC with developing and maintaining relevant guidelines. The guidelines it developed were based extensively on research by the European Audetel (Audio Described Television) Consortium, a now-defunct group that conducted extensive research in the mid-1990s regarding the technical, artistic, logistical, and economic issues associated with audio description. The ITC document serves as an excellent resource that explores in considerable detail (with practical examples) use of the present tense; prioritizing information; giving additional information; signposting or anticipating the action; stating the obvious; highlighting sound effects; use of proper names and pronouns; adjectival descriptions; use of adverbs; colors and ethnic origins; use of verbs; logos and opening titles; cast lists and credits; and other considerations.
As educational entities prepare to add audio description to video content, these resources will help ensure consistency and high quality.
-  DVS FAQ
-  Guidelines for Audio Description
-  Guidance on Standards for Audio Description