Display manager and login screen

In short: The display manager is the program that shows the graphical login screen, checks users and starts the chosen session (desktop).

In more detail: It runs as a system service before the user session. At the bottom or top you often choose the session (for example “Plasma (Wayland)” or “GNOME on Xorg”).

In detail

• GDM: the manager of GNOME.
• SDDM: the manager of KDE Plasma, customisable with themes.
• LightDM: light, common in Xfce, Mint and Lubuntu, with exchangeable “greeters”.
• Without a manager: log in on the text console and then startx or start the compositor.
• Session files: /usr/share/xsessions and /usr/share/wayland-sessions.
• Automatic login: possible in the manager’s settings but insecure.
• Switching: sudo systemctl enable sddm changes the manager (with systemd).

See also: X11 and Wayland, systemd and init systems