A peer-to-peer (P2P) application allows a device to act as both a client and a server within the same communication, as shown in the figure. In this model, every client is a server and every server a client. Both can initiate a communication and are considered equal in the communication process. However, P2P applications require that each end device provide a user interface and run a background service. When you launch a specific P2P application, it loads the required user interface and background services; afterward, the devices can communicate directly.

Some P2P applications use a hybrid system where resource sharing is decentralized, but the indexes that point to resource locations are stored in a centralized directory. In a hybrid system, each peer accesses an index server to get the location of a resource stored on another peer. The index server can also help connect two peers, but after connected, the communication takes place between the two peers without additional communication to the index server.

P2P applications can be used on P2P networks, client/server networks, and across the Internet.