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
LiveType
Cinema Tools
Compressor
Shake
Soundtrack
Logic Audio
iDVD
iTunes
Backup (.Mac)
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.