What is an Image Alt Text?

[Zora2012]

The ALT tag adds a text description to an image on a Web page, and should be used for all images, graphical bullets, and graphical horizontal rules. The alt text within the ALT tag should let the user know what an image’s content and purpose are.
Alt text is accessed by screen reader users to provide them with a text equivalent of images. In visual browsers, the alt text is displayed when an image is broken, or when all images have been disabled. Using ALT tags is also beneficial to users on Palm Pilots or low-bandwidth connections, where images are slow to download.