In computer science peer-to-peer networks are computer networks with a decentralized architecture consisting of individual nodes (the "peers") that (- in distinction from "traditional" cetralized concepts -) play essentially equal roles as they can be servers and clients at the same time.
On the one hand the concept itself is based on sharing itself because it uses the cumulative bandwidth of network participants, on the other hand it makes for useful sharing applications. It can be used for tools for sharing of digital content, ressources like internet bandwith and more.
Peer-to-peer ("P2P") technology is a common example for collaboration technology and the spirit of sharing connected to the internet in general.
Other prominent uses of peer-to-peer technology are the application for creating censorship resistant networks and for anonymous internet access/communication.