To create accessible web pages, you need to mark up page elements with information for assistive technologies, such as screen readers. For example, each image on a page should be given a text equivalent, called alternative text (or alt text), which a screen reader conveys to users in place of the image.
Alternative text appears on the image placeholder while the image is downloading from the web; it also substitutes for graphics that fail to download. In some newer versions of browsers, the text appears next to the pointer as a ToolTip.
Adding alternative text to a slice or hotspot
Exporting HTML from Photoshop
Adding alternative text in an external application