There are many useful plugins to create nice tooltips.
I know two of them that use the jQuery framework:
- mb-tooltip: [http://pupunzi.open-lab.com/mb-jquery-components/mb-tooltip/]
- jquery-plugin-tooltip: [http://bassistance.de/jquery-plugins/jquery-plugin-tooltip/]
You have to surround your keywords with a span element and a class to apply the jQuery selector. Maybe it's preferable to query for keyword presence server-side creating the ad-hoc html code for displaying the right tooltips, otherwise you have to create a tooltip in an AJAX way, handling the mouse hover event on the keyword.