1. X Display Manager - users need only to type in their username and password at a friendly prompt, and they are dropped directly into the graphical environment. This is simple and easy to use, and is seen frequently in college campuses, cyber cafes, business environments, anywhere you have users not necessarily familiar with Unix to any great extent. From Linux Guide @FirstLinux

2. X display manager xdm manages a collection of X servers, which may be on the local host or remote machines. It provides services similar to those provided by init, getty, and login on character-based terminals: prompting for login name and password, authenticating the user, and running a session. xdm supports XDMCP (X Display Manager Control Protocol) and can also be used to run a chooser process which presents the user with a menu of possible hosts that offer XDMCP display management. If the xutils package is installed, xdm can use the sessreg utility to register login sessions to the system utmp file; this, however, is not necessary for xdm to function. From Debian 3.0r0 APT