Many people can use an OS X system simultaneously for a variety of things.

Smooth and fair sharing of the computer between applications and users is ensured, even under the heaviest of loads. Preemptive multitasking with dynamic priority adjustment. BSD provides many advanced features, including the following: BSD variants in general are derived (sometimes indirectly) from 4.4BSD-Lite Release 2 from the Computer Systems Research Group (CSRG) at the University of California at Berkeley. The BSD portion of the OS X kernel is derived primarily from FreeBSD, a version of 4.4BSD that offers advanced networking, performance, security, and compatibility features.