The client-server architecture is a system that realizes a distribution of both processing and data. The central element of the architecture is the server. The server can be considered both from a logical point of view and from a physical point of view. From the physical point of view—hardware—a server is a machine dedicated to running a software server.

From a logical point of view, a server is software. The server, as a logical process, provides services to other processes that take on the role of requester or client. Generally, the server does not send the results to the requester until the results are requested by the client.

A feature that distinguishes the client from its server is that the client can initiate a transaction with the server, while the server can never initiate a transaction with the client on its own initiative:

In fact, the specific tasks of the client are to start transactions, request specific services, notify the completion of the service, and receive results from the server, as shown in the preceding diagram.