Tool Tips or Screen Tips

In many websites when the curser hovers over a link a small box displays a message. This message is called a tool tip or a screen tip.

The image map on the title page of the Round Australia Expedition online exhibition uses tool tips to indicate the location of the links to the eight sections of the expedition description.

The tool tip is created using the title attribute within the image tag.

An example would be title="map showing route of the expedition".
The alt attribute (alternative text) in some browsers will also provide a box displaying a message. This occurs in Internet Explorer but not in Mozilla Firefox. Alternative text is designed to be used to describe an image when the image is not displayed on the web page.

An example of the use of alternative text.

If the aim is to display tool tips on a web page it is recommended that both the title attribute and the alt attribute are used.

The following article - The Title Attribute - provides further information http://webdesign.about.com/od/htmltags/a/aa101005.htm