A Multi-user operating system is a computer operating system which allows multiple users to access the single system with one operating system on it. In the multi-user operating system, different users connected at different terminals and we can access, these users through the network. .
A multi-user operating system (OS) is one that can be used by more than one person at a time while running on a single machine. Different users access the machine running the OS through networked terminals. The OS can handle requests from users by taking turns among connected users.
You may not think of your personal computer as a multi-user system, because only you use it. However Unix systems (which OS X operating system is part of) are designed to be capable for multi-user functionality. This means that it is possible for another user to connect to your operating system and share your computer's resources (with your permission, of course).
A single-user operating system cannot share resources with other users. However a multi-user operating can.