XDM (in full, the X Display Manager) is the default display manager for the X Window System. It is a bare-bones X display manager. It was introduced with X11 Release 3 in October 1988, to support the standalone X terminals that were just coming onto the market.
Minimalistic, yet full-featured display manager that provides a fast, dialog-based login system without the overhead of the X Window System. Written in pure bash, CDM has almost no dependencies, yet supports multiple users/sessions and can start virtually any desktop environment...