The client -server software architecture model distinguishes client systems from server systems, which communicate over a computer network . A client-server application is a distributed system comprising both client and server software. A client software process may initiate a communication session, while the server waits for requests from any client.

Client/server describes the relationship between two computer programs in which one program, the client, makes a service request from another program, the server. Standard networked functions such as email exchange, web access and database access, are based on the client/server model.

The client/server model has become one of the central ideas of network computing. Most business applications being written today use the client/server model. So do the Internet's main application protocols, such as HTTP, SMTP, Telnet, DNS, etc. In marketing, the term has been used to distinguish distributed computing by smaller dispersed computers from the "monolithic" centralized computing of mainframe computers.

Source: Wikipedia