The last chapter discussed sockets from the standpoint of clients : programs that open a socket to a server that's listening for connections. However, client sockets themselves aren't enough; clients aren't much use unless they can talk to a server, and the Socket class discussed in the last chapter is not sufficient for writing servers. To create a Socket , you need to know the Internet host to which you want to connect. When you're writing a server, you don't know in advance who will contact you, and even if you did, you wouldn't know when that host wanted to contact you. In other words, servers are like receptionists who sit by the phone and wait for incoming calls. They don't know who will call or when, only that when the phone rings, they have to pick it up and talk to whoever is there. You can't program that behavior with the Socket class alone.
For servers that accept connections, Java provides a ServerSocket class that represents server sockets. In essence, a server socket's job is to sit by the phone and wait for incoming calls. More technically, a server socket runs on the server and listens for incoming TCP connections. Each server socket listens on a particular port on the server machine. When a client on a remote host attempts to connect to that port, the server wakes up, negotiates the connection between the client and the server, and returns a regular Socket object representing the socket between the two hosts . In other words, server sockets wait for connections while client sockets initiate connections. Once a ServerSocket has set up the connection, the server uses a regular Socket object to send data to the client. Data always travels over the regular socket.