I recently gave a presentation on Confluence plugins—what they do, how they work and why we have them—to an assorted bunch of Atlassian sales staff and customer advocates.
You can download my slides, as a Quicktime presentation. Even without me talking over them, I think they make sense. (In fact, they probably make more sense on their own, since I had to give this presentation at 9am, a time at which my brain is generally still demanding I go back to bed.)

Confluence Plugins Presentation