GDM, the GNOME Display Manager, handles authentication-related backend functionality for logging in a user and unlocking the user's session after it's been locked. GDM also provides functionality for initiating user-switching, so more than one user can be logged in at the same time. It handles graphical session registration with the system for both local and remote sessions . In cases where the session doesn't provide it's own display server, GDM can start the display server on behalf of the session.