Based on UNIX - Solid As a Rock
Don't let its elegant and easy-to-use interface fool you. Beneath the surface of Mac OS X lies an industrial-strength UNIX foundation hard at work to ensure that your computing experience remains free of system crashes and compromised performance. Time-tested security protocols in Mac OS X keep your Mac out of harm's way.
The most widely-sold UNIX-based operating system, Mac OS X offers a unique combination of technical elements to the discerning geek, such as fine-grained multithreading, Mach 3.0 microkernel, FreeBSD services, tight hardware integration and SMP-safe drivers, as well as zero configuration networking. Tiger's state-of-the-art kernel features improved SMP scalability and 64-bit virtual memory, while standards-based access control lists take UNIX permissions to the next level.
Command-line Environment
UNIX users will feel at home in Darwin, the robust BSD environment that underlies Mac OS X. That environment is accessible at any time from the Terminal application. You can also run commands that don't require arguments (such as top) by double-clicking them in the Finder. With the thousands of man pages included in Mac OS X, you can quickly find all your favorite UNIX tools.
UNIX Utilities and Scripting Languages
All of the standard UNIX utilities and scripting languages are included in Mac OS X: editors such as emacs, vim and even ed; file management tools such as cp, mv, ls and tar; shell scripts including bash (the default shell), tcsh (csh) and zsh. Tiger adds the korn shell so you can run scripts written for other operating systems more easily. And of course you can use scripting languages such as Perl, PHP, tcl, Ruby and Python, with native support for the popular Tcl/TK, TKInter and WxWidgets toolkits. Python users can also script the powerful Quartz compositing engine. Visit the Open Source page for more Open Source utilities in Mac OS X.
Universal Libraries
Mac OS X provides a robust set of optimized libraries, making it easy to port your existing UNIX code. For example, a standard, multithreaded C library (libc) includes support for such capabilities as reentrant variants of standard functions, facilitating the porting of thread-aware applications to Mac OS X. For applications that require non-Roman character sets, Mac OS X supports wide character datatypes (wchar_t and others). Tiger also supports UNIX/Linux portability APIs, including System V semaphores, so porting applications from versions of UNIX such as Linux and Solaris presents no problem. Tiger enhances the stellar cross-platform API support in Panther by adding powerful new Open Source libraries for XML transformations (libxslt) and data persistence (SQLite), as well as support for common UNIX services such as System V message queues.
Liftoff With launchd
Since Mac OS X rarely requires you to reboot, you'll hardly ever notice it, but Tiger takes less time to start up, thanks to launchd. Launchd provides faster startup through a unified framework for starting, stopping and managing daemons, and incorporates inetd, init, mach_init, System Starter and related services. Administrators have a single mechanism for auditing, configuring and setting resources limits on services.
Kernel
The Mach kernel augments standard virtual memory semantics with the abstraction of memory objects. This enables Mac OS X to manage separate application environments simultaneously and supply the following features:
Preemptive and cooperative multitasking.
Symmetric multiprocessing (SMP) augmented by support for multithreading.

Real-time support guaranteeing low-latency access to processor resources for time-sensitive media applications.