A server is a part of a distributed application that provides a service or services that are consumed by other parts of this application—clients. Clients communicate with the server in order to consume services provided by it.

Usually, a server application plays a passive role in the client-server communication process. During start-up, a server application attaches to a particular well-known port (meaning, it is known to the potential clients or at least it can be obtained by the clients at runtime from some well-known registry) on the host machine. After this, it passively waits for incoming requests arriving to that port from the clients. When the request arrives, the server processes it (serves) by performing actions according to the specification of the service it provides.

Depending on the services that particular server provides, the request processing may mean a different thing. An HTTP server, for example, would usually read the content of a file specified in the request...