A long description, created by using the longdesc attribute in HTML, is a method of providing equivalent text for non-text elements, such as images, objects, and applets. The text value for a long description is the URL where the long description is available. Because some browsers do not support the longdesc attribute, you should use both alternative text and long descriptions for Web elements that require a long description.
a hyperlink (to a page holding a longer description of an image) and a title is made up of text
a long description of an image
Like alt text, a longdesc is used to tell users who can't see graphics about an image. The longdesc element adds a link to a new page with a description of an image. For use when you need to give more information than you can in an alt text.
Similar to ALT text, the LONGDESC associates an image description with the code that places the image in the web page. This is in contrast to the D link, which simply places a link near the image.