The Display Managers: GDM and KDM
With the graphical login, your GUI starts up immediately and displays a login window with boxes for a username and password. When you enter your username and password and then press ENTER, your default GUI starts up. For most distributions, graphical logins are handled either by the GNOME Display Manager (GDM) or the KDE Display Manager (KDM). The GDM and KDM manage the login interface along with authenticating a user password and username and then starting up a selected desktop. If problems ever occur using the GUI, you can force an exit of the GUI with the CTRL-ALT-BACKSPACE keys, returning to the login screen (or the command line if you started your GUI from there). Also, from the display manager, you can shift to the command line interface with the CTRL-ALT-F1 keys and then shift back to the GUI with the CTRL-ALT-F7 keys.
When you log out from the desktop, you return to the display manager Login window. From the Options menu, you can select the desktop or window manager you want to start up. Here you can select KDE to start up the K Desktop, for example, instead of GNOME. The Language menu lists a variety of different languages that Linux supports. Choose one to change the language interface.
To shut down your Linux system, click the Shutdown button. To restart, select the Restart option from the Options menu. Alternatively, you can also shut down or restart from your desktop. From the System menu, select the Shutdown entry. GNOME will display a dialog screen with the buttons Suspend, Shutdown, and Reboot. Shutdown is the default and will occur automatically after a few seconds. Selecting Reboot will shut down and restart your system. KDE will prompt you to end a session, shutdown, or logout. (You can also open a Terminal window and enter the shutdown, halt, or reboot command, as described later; halt will log out and shut down your system.)