A system's display manager (DM) is a graphical login manager. It is the program that starts the X server and user sessions, presenting the user with a graphical login screen (greeter). As of 11.10, the default display manager is LightDM; previously, GNOME Display Manager (GDM) was used.
The main role of a display manager on an X Windows system is to provide a graphical login, starting the X server (X Org in Ubuntu) and presenting a "greeter" (a graphical login screen), so that the user session starts with a GUI. Without a DM, the user would have to log in on a tty and then, if X Org is not configured to start on login, they would have to issue a command (such as
startx) to enter the desktop environment.
LightDM is the default display manager in Ubuntu.