One feature of the Internet Explorer browser that many people miss when they start using Firefox is tool tips. Internet Explorer tends to display the alt text when hovering over images as a tool tip. But it's easy to get other browsers to display these types of tool tips as well, just use the title attribute on your images.
<img src="image.gif" alt="Shasta, my dog" title="Shasta, my dog" style="width: 100px; height: 100px;" />
The advantage of using the title attribute is that you can use it on more than just images. If you want a tool tip to show up when someone hovers over a link or even just a block of text, you can use the title to display that. However, tool tips are not widely supported on any element except images and links.
<a href="index.html" title="Go to my homepage">My Homepage</a>
