Classifications of P2P networks

P2P networks can be classified by what they can be used for:
file sharing
telephony
media streaming (audio, video)
discussion forums
Other classification of P2P networks is according to their degree of centralization.
In 'pure' P2P networks:
Peers act as equals, merging the roles of clients and server
There is no central server managing the network
There is no central router
Some examples of pure P2P application layer networks designed for file sharing are Gnutella and Freenet.
There also exist countless hybrid P2P systems:
Has a central server that keeps information on peers and responds to requests for that information.
Peers are responsible for hosting available resources (as the central server does not have them), for letting the central server know what resources they want to share, and for making its shareable resources available to peers that request it.
Route terminals are used as addresses, which are referenced by a set of indices to obtain an absolute address.
e.g.
Centralized P2P network such as Napster
Decentralized P2P network such as KaZaA
Structured P2P network such as CAN
Unstructured P2P network such as Gnutella
Hybrid P2P network (Centralized and Decentralized) such as JXTA (an open source P2P protocol specification