A login manager is a login system for Unix and Unix-like operating systems. It comprises a login daemon, a login user interface, and a system for tracking login sessions. When a user tries to log in, the login manager passes the user's credentials to an authentication system.
Since an X display manager is a graphical user interface for login, some people use the terms display manager and login manager synonymously.
systemd, an init daemon for Linux, has an integrated login manager; its login daemon is called logind. systemd's login manager is an alternative to ConsoleKit, which is no longer maintained. A forked version of ConsoleKit called ConsoleKit2 is however available.
References
Login manager Wikipedia(Text) CC BY-SA