Multi-user systems
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. [2].
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.[3]
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.