You can start X sessions in a variety of ways. The Ubuntu installer sets up the system to have Linux boot directly to an X session using a display manager called LightDM, for Light(weight) Display Manager. This is an X client that provides a graphical login. After you log in, you use a local session (running on your computer) or, if the system is configured to do so, an X session running on a remote computer on the network.
Logging in via a display manager requires you to enter a username and password. You can also start X sessions from the command line. The following sections describe these two methods.