I chanced upon this today -

http://www.cs.tufts.edu/~nr/cs257/archive/jeannette-wing/pi.pdf

Kudos to Jeanette Wing for writing this FAQ in very simple language. There is a lot of great material about the Pi-Calculus out there but most of it is either pedagogical or addresses the research community - very little readable text that simply explains the lay-of-the-land.

If you are curious about concurrency and process calculi, the above is a pleasant read. On the other hand, if you are already well versed in the topic, there is probably nothing here that you don't already know.