Gonzalo Camarillo has just submitted draft-iab-p2p-archs-00, an overview on P2P architectures:

In this document we provide a survey of P2P (Peer-to-Peer) systems.The survey includes a definition and a taxonomy of P2P systems.  Thissurvey also includes a description of which types of applications canbe built with P2P technologies and examples of P2P applications thatare currently in use on the Internet.  Finally, we discussarchitectural tradeoffs and provide guidelines for deciding whetheror not a P2P architecture would be suitable to meet the requirementsof a given application.

The documents definition of a P2P system:

We consider a system to be P2P if the elements that form the system share their resources in order to provide the service the system has been designed to provide.

The elements in the system both provide services to other elements and request services from other elements.

and a real quote from the draft:

ZebraNet is a network used to track Zebras in the wild.

It contains zebras, so must be worth a read!