A server is a program running on the remote machine providing service to the clients. When it starts, it opens the door for incoming requests from clients, but it never initiates a service until it is requested to do so. A server program is an infinite program. When it starts, it runs infinitely unless a problem arises. It waits for incoming requests from clients. When a request arrives, it responds to the request, either iteratively or concurrently as we will see shortly.
A client is a program running on the local machine requesting service from a server. A client program is finite, which means it is started by the user (or another application program) and terminates when the service is complete. Normally, a client opens the communication channel using the IP address of the remote host and the well-known port address of the specific server program running on that machine. After a channel of communication is opened, the client sends its request and receives a response. Although the request-response part may be repeated several times, the whole process isfinite and eventually comes to an end.