The heart of Mac OS X is based on the Unix operating system. Unix was developed by AT&T in the early 1970s. In those days, computers were large and expensive, and Unix was intended as a way to share computing resources between multiple users at once. It was likely that an organization at that time could afford only one computer for all its members, and Unix provided a way for people to use that computer simultaneously without getting in each other's way.
Over the years, Unix development has split off into many distinct "flavors" of Unix, all headed up by different groups of people, all with somewhat different goals. BSD and Linux are two such examples. Each version of Unix shares some portion of the original vision and typically implements a common set of libraries and commands.
Unix is regarded as a robust operating system whose scalability and innate networking capability make it ideal for use as a server. In fact, most of the modern day Internet is powered by Unix servers of one version or another. It turns out that these features are also desirable in modern desktop operating systems. So it is no surprise when seeking to modernize the original Macintosh operating system, Apple turned to Unix.
Mac OS X's core operating system is a Unix flavor called Darwin. Like most Unix flavors, Darwin's source code is freely available, allowing interested parties to see exactly how the core operating system works. Apple maintains several resources for programmers ...