P2P (peer-to-peer) is a decentralized network where files can be shared directly between computers connected to each other via the Internet without any central servers. Each computer on the p2p network becomes a file server itself and is called a peer. All peers are equally privileged participants of the p2p network. They are both suppliers and consumers of resources.
To join the p2p network computers should have the Internet connection and a special p2p software. If your computer is connected to the p2p network, you can look for files on other computers in the network in the particular folders designated for sharing.
The P2P concept was popularized by file sharing systems at the end of the 20th century. The most famous example of the p2p network is Torrent. File sharing in Torrent network is organized with special programs called torrent clients (BitTorrent, uTorrent, etc.) that automatize exchange process between the peers.