The Client/Server Model is an architecture defining how several actors spread out over a network cooperatively behave is if they were one in the eyes of the user.

The user, always a program, requests a service from the system. The server, another program, fulfills the request and delivers the answer as a service to the user. It is a fundamental characteristic that the user doesn't know the whereabouts of the server(s). He merely knows the names of the servers.

In some cases the server needs help in order to fulfill a requested service. In such a case he acts as a user and requests a service from another server. Typically a server can do calulations, the logics, the application for a user, but he possibly does not store the data to be calculated upon. He therefore requests them from a data managemnt server that manages data needed by the applications.

If the first server is capable of honouring the request from the user, we call it a 2-tier Client/Server System. In case of any need for further assistance we call it a 3-tier system.

Please notice that tier is pronounced as tear! As in "Tears of joy."