Pop-up info window on links

[windows]

I've seen a webpage before when, when you move the mouse over a link, a little info box pops up, much like what happens with the "alt" tag in an image...how do I do that? I need to use it for a new page I'm making. Any input would be appreciated; I'm making the page as a surprise for someone and I'd like to finish it ASAP! Thanks.<!--content-->http://www.dynamicdrive.com/dynamicindex5/popinfo3.htm<!--content-->