From user to user. Peer-to-peer implies that either side can initiate a session and has equal responsibility. Peer-to-peer is a somewhat confusing term, because it has always been contrasted to a central system that initiates and controls everything. But in practice, two users on a peer-to-peer system often require data from a third computer. For example, the infamous Napster file sharing service was always called a "peer-to-peer network," but its use of a central server to store the public directory made it both centralized and peer-to-peer.|
The two major categories of peer-to-peer systems are for file sharing (see peer-to-peer network) and CPU sharing (see grid computing). In a wireless network, a peer-to-peer architecture is called an "ad hoc" network (see mobile ad hoc network).
There are many applications and services that claim to be peer-to-peer. Visit www.openp2p.com and click on the "P2P Directory" for a comprehensive list of resources, articles and blogs about the subject.
O'Reilly's book, "Peer-To-Peer; Harnessing the Power of Disruptive Technologies" is a collection of potent articles by peer-to-peer experts. Edited by Adam Oram, it covers every major pioneering P2P effort, including Napster and SETI, and explains all the architectures and ramifications of the subject. This is essential peer-to-peer reading. (O'Reilly & Associates, Inc., 2001, ISBN 0-596-00110-X).