Aside from the "alt" (alternate) or "longdesc" (long description) attributes being defined as required in web standards, which will make your page valid xhtml... Alt is the text that will display in place of your image when it cannot be loaded. The best example is the "user posted image"s you see a lot of forums. "User Posted Image" is the alt text. Its use would be... Say your navigation bar was comprised solely of images. Say somehow those image become broken. The images will no longer display, thus rendering your navigation a complete mystery (to the average web user). Will the proper use of the alt att., people would see text where the images were, things like "home", "forums", etc. Hope that helps.
