A peer-to-peer (or P2P) computer network uses diverse connectivity between participants in a network and the cumulative bandwidth of network participants rather than conventional centralized resources where a relatively low number of servers provide the core value to a service or application. P2P networks are typically used for connecting nodes via largely ad hoc connections. Such networks are useful for many purposes. Sharing content files (see file sharing) containing audio, video, data or anything in digital format is very common, and real time data, such as telephony traffic, is also passed using P2P technology.

A pure P2P network does not have the notion of clients or servers but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client-server model where communication is usually to and from a central server. A typical example of a file transfer that is not P2P is an FTP server where the client and server programs are quite distinct: the clients initiate the download/uploads, and the servers react to and satisfy these requests.

In contrast to the above discussed pure P2P network, an example of a distributed discussion system that also adopts a client-server model is the Usenet news server system, in which news servers communicate with one another to propagate Usenet news articles over the entire Usenet network. Particularly in the earlier days of Usenet, UUCP was used to extend even beyond the Internet. However, the news server system acted in a client-server form when individual users accessed a local news server to read and post articles. The same consideration applies to SMTP email in the sense that the core email relaying network of Mail transfer agents follows a P2P model while the periphery of e-mail clients and their direct connections is client-server. Tim Berners-Lee's vision for the World Wide Web, as evidenced by his WorldWideWeb editor/browser, was close to a P2P network in that it assumed each user of the web would be an active editor and contributor creating and linking content to form an interlinked "web" of links. This contrasts to the more broadcasting-like structure of the web as it has developed over the years.

Some networks and channels such as Napster, OpenNAP and IRC serving channels use a client-server structure for some tasks (e.g. searching) and a P2P structure for others. Networks such as Gnutella or Freenet use a P2P structure for all purposes, and are sometimes referred to as true P2P networks, although Gnutella is greatly facilitated by directory servers that inform peers of the network addresses of other peers.

P2P architecture embodies one of the key technical concepts of the Internet, described in the first Internet Request for Comments, RFC 1, "Host Software" dated April 7, 1969. More recently, the concept has achieved recognition in the general public in the context of the absence of central indexing servers in architectures used for exchanging multimedia files.