An X Display Manager presents a graphical login screen to X users. Often an XDM will allow the user to select a desktop environment or window manager to be for their login session. Some X Display Managers are xdm, gdm (Gnome Display Manager), and kdm (KDE Display Manager).
XDMCP allows XDM to process logins for users remote to the machine that XDM is running on; login sessions will be run on the machine running XDM. For example, at a university you may use XDMCP to login to an X session running on an engineering department computer from your dorm room.
See Also: X Display Manager.
Some SSH programs like OpenSSH provide a feature where in addition to the normal text channel opened between the local and the remote host another encrypted channel is opened for the communication between the X11 client and the X11 server. The SSH server will act as proxy-server for the X11 clients and will forward all communication to the X11 server.