A Login Manager (aka. display manager) is a graphical login GUI, usually intended for a X Window System oriented workstation or desktop. It allows users to log into Unix/Linux using X Window System without ever needing to see the CLI.
Controlling Login Managers
A user with enough permission can kill a display manager and return to the CLI like so...
- Any Distro: # killall gdm
- Debian: # /etc/init.d/gdm stop