Xdm manages a collection of X displays, which may be on the local host or
remote servers. The design of xdm was guided by the needs of X terminals as
well as The Open Group standard XDMCP, the X Display Manager Control Protocol.
Xdm provides services similar to those provided by init, getty and login on
character terminals: prompting for login name and password, authenticating the
user, and running a session.