LightDM is a 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.
• Supports different display technologies (X, Mir, Wayland ...).
• Lightweight - low memory usage and fast performance.
• Guest sessions.
• Supports remote login (incoming - XDMCP, VNC, outgoing - XDMCP, pluggable).
• Comprehensive test suite.

Releases are synchronised with the Ubuntu release schedule and supported for the duration of each Ubuntu release. Each release is announced on the mailing list.

The core LightDM project does not provide any greeter with it and 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.

Questions should be asked on the mailing list. All questions are welcome.

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