You enter your term into a glossary and provide a description (that's shown in the tooltip) and optionally an extended description (only shown on the glossary page). You can use html in your entries. A content plugin takes care of automatically highlighting those terms in your article.
What I found useful is that you can specify tags, (e.g, h1, h2) where you don't want a definition highlighting. It matches glossary entries very well. If your glossary entry is singular but in your article you add an 's' to make it plural, you'll still get a match. You can also use a plugin tag to disable the feature on certain articles or intro texts.
If you're buying the component - pay for the support, it's worth it. I had a couple of issues (that required bug fixes) and the developer was great in responding to and resolving my problems. Couldn't ask for more.
The component element is the message store. Then you configure instances of the module to display the message. I made use of the conditional cookie feature to display the message to the visitor until they closed it. Then Sticky Message remembers that they've already seen the message.
I did have one issue. I needed to increase the expiry time for a conditional cookie. The support was very quick - same day - and very helpful.
A worthwhile component for the price that does what it says.
We upgraded to the professional version because you can include databases and folders/files outside of Joomla as part of the backup. Plus you have more options for excluding items from the backup.
Akeeba just works. Its the best way to backup/restore or move/copy your Joomla site.