The KDE Display Manager is a ``drop-in'' replacement for xdm, the X Display Manager and a little utility. kdmdesktop is to draw nice backgrounds. Kdm is superior to xdm in several ways:
Kdm supports session arguments in a userfriendly way (xdm doesn't tell the user, which args it might generate).
Kdm has an optional ``shutdown''-button (much like NT's login box).
Kdm has an optional graphical view of users on the system (idea stolen from IRIX' login box).
Kdm looks ways better than xdm ;-)
Kdm can be configured with a graphical user interface (a part of the KDE control center)