A common structure for distributed systems—computing systems in which more than one computer is linked together--is one in which software or computing tasks are split between a server and a client. A client sends requests to a server according to some protocol, asking for information or action, and the server responds. This is analogous to a real-life customer who sends an order for goods or services to a supplier, who then provides the goods and an invoice. This interaction of customer and supplier involves a certain amount of formal paperwork; likewise, a protocol must be used by client and server computers in order to communicate effectively.

A client-server system may have either one central server or many (distributed) servers. A given machine may even act as both client and server for different applications. Clients and servers may be placed independently on a communications network...