Think of it in terms of a print server. It's a long-running process that other processes connect to, in order to request a service. It handles the "impedance mi

Think of it in terms of a print server. It's a long-running process that other processes connect to, in order to request a service. It handles the "impedance mismatch" between the clients and the hardware it controls---in the case of a print server, it demultiplexes requests and translates them into something the printer can deal with; in the case of X, it displays the information as requested by the client and provides notifications of events as requested by the client.