These "tool-tips" can give valuable help to your visitors, and make your page look neater - you don't need to have long explanations on display. By including them in a title tag, your visitors will see them when they need them.
They'll work in Internet Explorer version 4 onwards. Netscape version 4 and later should show them correctly, but I haven't been able to test all versions.
Dianne Reuby is co-author of the e-book "First Website Builder". Dianne created and runs the First Web Builder site, dedicated to providing ebooks and tips for new webmasters. Visit FWB at http://firstwebbuilder.co.uk/