Starting in 2000 with Mac OS X, Apple’s desktop operating system has been Unix-based. Why is this significant? Many developers were already familiar with the Unix operating system. With a Unix base, coders could use standard Unix commands in the command line, there was greater code portability, and Mac OS X got all of the associated benefits of Unix. For example, Unix is architected in a modular fashion, which can be beneficial when it comes to system security.
Scott Forstall’s departure (Why Apple (and You) Might Miss Scott Forstall)