X uses a client-server protocol, the X protocol. The

server is the computer or X terminal with the screen,

keyboard, mouse and server program and the clients are

application programs. Clients may run on the same computer

as the server or on a different computer, communicating over

Ethernet via TCP/IP protocols. This is confusing because

X clients often run on what people usually think of as their

server (e.g. a file server) but in X, it is the screen and

keyboard etc. which is being "served out" to the applications.