In Mac OS X, what is fast user switching, and how do I use it?

In Mac OS X, a feature called fast user switching allows you to simultaneously log into two or more OS X accounts on one computer, and switch quickly between them. To enable and use fast user switching:

1. From the Apple menu, select System Preferences... .
   
   
2. In System Preferences, from the View menu, select Users & Groups (Mac OS X 10.7 and later) or Accounts (Mac OS X 10.6 and earlier).
   
   
3. If the padlock in the bottom left-hand corner of the window is closed, click it and authenticate with your administrator login information.
   
   
4. On the bottom left side of the window, click Login Options.
   
   
5. On the right side of the window, check Show fast user switching menu as: (Mac OS X 10.6 and later) or Enable fast user switching (Mac OS X 10.5 and earlier). If a sheet then drops down, click OK. Choose one of the options to determine how the user switching icon appears in the menu bar.
   
   
6. To log into a different account, click the fast user switching icon in the menu bar, and from the pull-down menu that appears, select the account you wish to log into.
   
   
7. When prompted, provide the password for the account. It may take a moment for the login process to complete. After this, the screen will rotate and the other account will become dominant.
   
   
8. To access the first account, or to log into a different account, repeat steps 6 and 7, selecting the appropriate account.

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