A client/server networking model is a model in which computers such as servers provide the network services to the other computers called clients to perform some user based tasks.

Client

A client is a program that runs on the local machine requesting service from the server. The client program is a finite program which means that the service started by the user terminates when the service is completed

Server

A server is a program which runs on the remote machine, generally with high capacity, providing services to the clients. When the client requests for a service, then the server opens the door the door for the incoming requests. The server never initiates the service.

A server program is an infinite program which keeps on running generally in the back end. The server waits for the incoming requests form the clients. When the request  arrives at the server, it just responds to the request.