The modern peer-to-peer concept was popularized by file-sharing systems, such as the music-sharing application Napster, which appeared in 1999. The peer-to-peer movement allowed millions of internet users to directly connect, form groups, and collaborate with each other to function as user-created search engines, virtual supercomputers, and file systems. This model of network arrangement differs from the client-server model, where communication is usually to and from a central server.
Today P2P services have moved beyond purely internet services, though they are mostly thought of as at least internet-based. Peer-to-peer services involve activities that range from simple buying and selling to those that are considered part of the sharing economy. Some peer-to-peer services don't even involve a paid transaction by the users at all, but they bring together individuals to work on joint projects, share information, or communicate without direct intermediation. These kinds of P2P services may be operated as free nonprofit services or generate revenue by advertising to users or by selling user's data.