LightDM is a lightweight, cross-desktop display manager. A display manager is a daemon that:

• Runs display servers (e.g. X) where necessary.
• Runs greeters to allow users to pick which user account and session type to use.
• Allows greeters to perform authentication using PAM.
• Runs session processes once authentication is complete.
• Provides remote graphical login options.

Key features of LightDM are:

• Cross-desktop - supports different desktop technologies (X, Wayland, Mir, etc)
• Lightweight - low memory usage and fast performance
• Supports remote login (incoming: XDMCP and VNC; outgoing: XDMCP and pluggable)
• Supports guest sessions
• Has a comprehensive test suite

The core LightDM project does not provide any greeter with it; you should install a greeter appropriate to your system. Popular greeter projects are:

• LightDM GTK+ Greeter - a greeter that has moderate requirements (GTK+).
• LightDM KDE - greeter used in KDE (Qt)
• LXQt Greeter - greeter used in LXQt (Qt)
• Pantheon Greeter - greeter used in elementary OS (GTK+/Clutter).
• Unity Greeter - greeter used in Unity.
• WebKit2 Greeter - greeter that can be themed using HTML/CSS/Javascript
• Run with no greeter (automatic login only)
• Write your own...

LightDM configuration is provided by the following files:

/usr/share/lightdm/lightdm.conf.d/*.conf
/etc/lightdm/lightdm.conf.d/*.conf
/etc/lightdm/lightdm.conf

System provided configuration should be stored in /usr/share/lightdm/lightdm.conf.d/. System administrators can override this configuration by adding files to /etc/lightdm/lightdm.conf.d/ and /etc/lightdm/lightdm.conf. Files are read in the above order and combined together to make the LightDM configuration.

For example, if a sysadmin wanted to override the system configured default session (provided in /usr/share/lightdm/lightdm.conf.d) they should make a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf with the following:

[Seat:*]
user-session=mysession

Configuration is in keyfile format. For most installations you will want to change the keys in the [Seat:*] section as this applies to all seats on the system (normally just one). A configuration file showing all the possible keys is provided in data/lightdm.conf.

LightDM can be configured to run an external shell script to setup displays.

If an display setup script is used, it must be:

• Located under /usr/share
• Owned by the user lightdm and group lightdm
• It cannot print or log to any destination not accessible to LightDM

To test a configuration:

• Install xserver-xephyr: sudo apt install xserver-xephyr
• Run the test as user lightdm: sudo -u lightdm lightdm --test-mode --debug

Put the shell script reference in the LightDM configuration:

[Seat:*]
display-setup-script=/usr/share/example_display_setup_script.sh 

Stack Overflow and Ask Ubuntu are good sites for frequently asked questions.