Fast user switching is a feature Microsoft implemented within Windows XP Home Edition and Windows XP Professional that allows users to switch between users without having to log out of their user accounts. It allows applications to remain open and in the same state, despite someone new logging in. This allows the previous user to get back to his or her tasks more quickly.
Techopedia explains Fast User Switching
Windows allows multiple users to have their own profile set-ups associated with their account. These profiles are usually password protected, and their settings, files and other information are set up to meet each individual user's needs. Setting up different profiles allows multiple users to share one computer. However, there is often a delay when logging in and out of different accounts. This is where fast user switching comes in.
Fast users switching allows multiple users to be logged in simultaneously and switch between their open accounts while other applications are running and network connections are preserved.
This definition was written in the context of Microsoft Windows