Glossary words and tooltips

[cinq]

You know how some site have articles which have certain words highlighted with tooltips to give a short definition of the term ?

Where they draw the description perhaps from a database of terms and their respective definitions?

How can that be acheieved ?
How do they search an entire article ( word for word ) to compare with a database of known terms and definitions ?

I am thinking of doing a hack such that on one hand, you have a glossary on your forums, and at the same time, it links up with the posts in the forum where terms that are in posts/threads which are also in the glossary database will be highlighed with a tooltip to show the term definitions.

Do advise