The client-server model, or client-server architecture

       The client-server model (client-server architecture) is an approach to computer network programming in which computers in a network assume one of two roles: The server selectively shares its resource, and the client initiates contact with a server in order to use those resources.
The client-server model is prevalent in computer networks. Email, Network printing, and the world wide web all apply the client-server model.



How clients and servers communicate?
 
        Clients and servers exchange massages in a request-response messaging pattern: The client sends a request, and the server returns a response. This exchange of messages is an example of inter-process communication. To communicate, the computer must have a common language, and they must follow rules so that both the client and the server known what to expect. The language and rules of communication are defined in a communication protocol. All client-server protocols operate in the application layer.
A server may receive requests from many different clients in a very short period of time, Because the computer can perform a limited number of tasks at any moment, it relies on a scheduling system to prioritize incoming requests from clients in order to accommodate them all in turn.