You can do two things, if you want a real popup browser window to open,
you'll need to do this with JavaScript. There are already lots of good free
scripts out there so just search
http://javascript.internet.com/navigation/
for some nice ones.
The other thing you can do is to create a link around the word and set the
link's title attribute to whatever text you want. This will show a tooltip
effect for a few seconds like so:
<a href="#" title="This is some information">A Word</a>
Of course, the downside is since it has to be a link for this to
work users may get confused by the link. Using the popup browser method is
probably better, although it still requires you to create a link to do it,
at least id does something noticable for the user. Just be warned that some
of the more aggressive pop-up blockers may block the use of this feature.
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Frontpage