Switching User in Mac OS X

In Mac OS X, there 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. Unlike Windows Vista or Windows 7, this feature is not turned on by default on Mac OS X.

Here are the steps enable Fast User Switching:

1. From the Apple menu, select System Preferences... .
2. In System Preferences, from the View menu, select Accounts
3. If the padlock in the bottom left-hand corner of the window is closed, click it and authenticate with an administrator account.
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) or Enable fast user switching (Mac OS X 10.5 and earlier). If a sheet then drops down, click OK. Choose Name, Short name, or Icon 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.

The drawback of this feature is that each user logged into a system eats up memory and the more applications open for each user, the more likely the system will swap stuff into virtual memory and system performance will suffer.