Display managers work to initiate the X Window System. X.Org is an open-source implementation of the X Window System protocol and the Xorg Desktop Environment is discussed as it is the most widely used implementation on Linux systems today, providing the actual software (the X server) that interprets the X11 protocol to render graphical interfaces on screens.
X Display Manager (XDM)
An early display managers created to serve the X Window System.
GDM (GNOME Display Manager)
A display manager made for the Gnome Desktop Environment.
LightDM
A cross-desktop display manager that works well with desktop environments such as Cinnamon, LXDE, Xfce, and Unity.
SDDM (Simple Desktop Display Manager)
A display manager for the KDE Plasma desktop environment. It uses the Qt (Qt Meta-Object Language) to provide a theme and customization.