Long descriptions (longdecs) are used much like the alt text: they provide site information in a text format. Long descriptions describe more detailed information about your graphics or multimedia presentations.
For example, charts or graphs that show a lot of data should use long descriptions to explain more elaborately what it is you are displaying. Generally, a long description is a link to a text file that provides the end user with information that the user of a screen reader will hear.
Longdecs are important because they provide the end user a more elaborate description the details of an image, Java applet, flash file, video file, audio file, plug-in, chart, figure, or graphics used in the web page.
Use the same procedure as inserting alt text to insert a long descritpion or a link to the long description text file you prepared for the end user who uses a screen reader to interface with your web site. See Alt Text
* For an expanded explanation on this procedure, view W3C Guideline 12: Provide context and orientation information.