Fast user switching allows you to switch between user accounts while other user accounts are still open.
In Mac OS X 10.3, FUS was enablable. It it enabled in System Prefernces -> Accounts. To switch, you select the users name from the menu in the upper right-hand corner (the User menu).
Note that there are peripherals and applications which do not work if you use Fast User switching– usually, they will not allow a second user to use the application if another user is already using it. Also, only one person can use Classic at a time on the computer.
Some Apple applications incompatiable with Fast User switching include (see KB #25619)
Final Cut Pro
Final Cut Express
DVD Studio Pro
iMovie can’t control an attached camera if the camera is already in use by iMovie or another application in a different user account.
Also, Palm/HotSync typically cannot operate if being used by another user.