ALT, which works just like <IMG ALT="...">, indicates the alternative text to use for the link if the browser does not display the image (for example if the user is blind and uses a text browser). If you decide to put an imge map in your web page, don't look like an amateur and leave out alternate text.
For example, the following <AREA ...> tag indicates that the alternate text is Contacts:
Some visual browsers do not utilize the ALT attribute but make user the TITLE attribute to display a prompt over the area, so it's a good idea to use both: