To have a tooltip bubble when mouse hovers over, use the “title” attribute in your link, like this:
<a href="http://example.com/" title="this is some nice link">somelink</a>
Example: hover mouse over me (don't move you mouse for a second)
If you want to have links and image in your tooltip, or control the size or color of the bubble, use Erik Bosrup's overLIB at: http://www.bosrup.com/web/overlib/.
Example: Please mouse over me..
Here's the code:
<a href="javascript:void(0);" onmouseover="return overlib('yes yes yes.');" onmouseout="return nd();">Please mouse over me.</a>.
Here's a tutorial about using overLIB: http://overlib.boughner.us/plugins/bubble_commands.html