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A server is a system (hardware and software) that responds to requests across a computer network to provide, or help to provide, a network service. Servers can be run on a dedicated computer, which is also often referred to as 'the server', but many networked computers are capable of hosting servers. In many cases a computer can provide several services and have several servers running.
Servers operate within a client-server architecture, servers are computer programs running to serve the requests of other programs, the clients. Thus, the server performs some task on behalf of clients. The clients either run on the same computer, or they connect through the network. In the context of Internet Protocol (IP) networking, a server is a program that operates as a socket listener.[1]
Servers often provide essential services across a network, either to private users inside a large organization or to public users via the Internet. Typical computing servers are database server, file server, mail server, print server, web server, gaming server, or some other kind of server.
Numerous systems use this client / server networking model including Web sites and email services. An alternative model, peer-to-peer networking enables all computers to act as either a server or client as needed. http://en.wikipedia.org/wiki/Server_(computing)