From AMule Project FAQ
Peer-to-Peer (also known as P2P or Peer-2-Peer) network refers to any network that does not have fixed clients and servers, but a number of peer nodes that function as both clients and servers to the other nodes on the network. This model of network arrangement is contrasted with the client-server model. Any node is able to initiate or complete any supported transaction. Peer nodes may differ in local configuration, processing speed, network bandwidth, and storage quantity. Popular examples of P2P are filesharing-networks.
This text has been taken from the Free-Definition database (here) under the GNU-FDL puclic license it is subjected to.