A feature of Microsoft's Windows XP operating system, which lets one user log on to a computer currently in use by another user. Each user's desktop, documents and settings stay open during this process. This is convenient for anyone who needs to get to something without interrupting whoever was using the computer at the time.
It works somewhat like Microsoft's Terminal Server, where multiple users can use a single computer with a remote desktop client. Only the "client" runs on the same machine.
Many applications that run in the background do not work with fast user switching, because they need exclusive access to certain resources. The most egregious examples include printer monitoring programs that display an icon beside the clock in the Taskbar. If you have such applications, ask the developer to correct them, as fast user switching support is a requirement of anything Designed for Windows XP. Alternately, you can turn this feature off in the User Accounts control panel, which also frees about 24 MB of memory for other applications.1
Fast user switching is not the same as the su command available in POSIX systems, nor is it the same as the "Run As" feature available in Windows since Windows 2000. These features run applications with the same desktop and other settings as the person using su or Run As.
1. Thanks to QuantumBeep for the heads-up. | 
