A personal computer is designed for one user working at a time. In other words, the operating system does not have to worry about processing input from several users who may be working at the same time.
As a multi-user operating system, the UNIX operating system must be able to handle more than one task at a time. On a UNIX system, several terminals might be connected to the computer and the operating system manages access to the computer by a community of users. This kind of operating system allows the computer to perform many functions at the same time.