The closest I've been able to come up with as a short definition is that portlets are sort of the MDI interface for a web page. Basically, you can define multiple portlets and then "lay them out" on the web page. Each portlet is supposed to have a common "tab", with controls that allow you to minimize, maximize or restore it. Each portlet communicates with a servlet-like class that is able to render itself on request.
The idea is to have independent entities on the page, like those pages where there's a little stock ticker, and you can hit the "update" button on the ticker and it updates itself.
The mechanics are a little hazy to me yet, but I got
Tomcat and JetSpeed up and running in about 10 minutes, so I'll know more soon (I hope).
Joe