When you turn on your computer, you see the operating system load. Once the system has started, the computer will open the GNOME user interface. GNOME runs atop the operating system and provides a Graphical User Interface (GUI) for the user to interact with. The login screen is the first screen a user will see when GNOME starts.
Types of login screen 
There are two types of login screen, standard and graphical. GNOME allows the users to select a login theme to customize their machine. Different operating systems provide different "preferred" graphical themes.
Some login themes include a "face browser", which lets the user visually select their user name, while others simply provide text boxes to enter a user name and password.
The graphical login screen often provides controls for shutting down or restarting the computer, and for configuring the login theme.
GNOME requires you to enter your user name and password on the login screen. Either type in your user name in the box, or double-click your face. Then type in your password and press the enter key. If you entered you password correctly, GNOME will display a welcome screen and open the main desktop.
Nested login 
Nested login lets you log in as another user without having to log out first. It displays their desktop in a Application Window rather than taking the whole screen.
Secondary login 
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