The User menu in the upper rightmost corner is identified by a bubble and the full name of the currently logged user. The bubble icon is meant to display the available or busy state in the stock IM client called Empathy.
Clicking the User menu various menu items are shown. Under the two IM status items there will be presented many others options.
My Account menu item will open the User Accounts dialog where you can configure new users and change the details of your account.
System Settings leads to the general control panel for many settings of GNOME 3.
Lock Screen will lock the current session so that the user password will be asked to unlock the session again.
Switch User will open a new X Server and will start a new login manager to allow another user to login.
Log Out item will show a confirmation dialog to close the current session and all the running programs and disconnects the user.
The Suspend menu item is meant to have a dual function: suspend the current session leaving the computer in stand-by mode or to power off the computer if the ALT key is pressed.
Clicking My Account in the User menu the User Accounts dialog will be shown to let the user change various details like the image avatar shown in the login manager, or his full name, language for the user interface and the login password.
The lock button in the left corner allows the users with administrative privileges to create new or change and delete existing users.
The Automatic Login feature allows to set a user that will login without any request from the login manager, useful for shared computer where no authentication is needed. This feature doesn’t remove the user password but it will simply skip during the first login. The password will be still asked if later needed for other tasks.
The System Settings option in the User menu will open the central control panel from which many desktop and system preferences can be configured.
When the user chooses the Lock Screen option in the User menu the screen will fade to black and the session will be password protected. Whoever wants to use the desktop again has to insert the user password again, even if the user used the automatic login.
If the password is unknown, a different user can login by clicking the Switch User button. Obviously the new user credentials must be known.
Both the Switch User button in locked screen dialog and the Switch User in the User menu will create a new X Server, invoking the login manager (GDM is the default GNOME login manager) allowing you to insert the credentials for the login.
The Log Out option in the User menu will present a confirmation dialog, giving 60 seconds before the current session will be logged out automatically. When the log out occurs all the applications will exit (the saving of the unsaved documents is not guaranteed), the whole session will be closed, the user will be disconnected and the login manager will be shown again.
If the ALT button is held pressed while the User menu is shown, the Suspend option will be substituted by the Power Off… option which, when clicked, will show a confirmation dialog to Restart of Power Off the computer, while a counter will progress for automatic power off after 60 seconds.
The User menu offers a lot of different options to configure the preferences or protect and change user details. The most important option in this menu is obviously the last to allow you to suspend the computer in stand-by and to power off or restart it if needed.
Further information can be found in: GNOME Shell Design.