The ability to hover over an image or a link and make it display some text recently came to my attention.
If you want an actual popup box, you may be looking for some jQuery stuff.
For the simple option of hovering over a link, and having some text explaining the link a little more the option is simple. All you have to do is create a title element for the link.
So, this means you make something like:
<a href="http://yourLink.com" title="type the hover message you want here">link text</a>
So now you should get something like this.