Mac OS X [prounounced "O-S 10"] The Operating System created by combining Darwin, an Open Source kernel based on the BSD source tree and the Mach microkernel, with a custom Apple GUI. The Darwin kernel provides an extremely stable and flexible operating system which rivals many Unix implementations. OS X's users consider its GUI to be the most beautiful and functional in existence. It is a powerful combination, but analysts are still waiting to see the long term effects on the computer industry.
OS X is compatible with the previous versions of the Mac OS by using the Classic application, which runs Mac OS 9.x under emulation (the application runs in the background; any applications appear to run like OS X applications.
OS X can run many BSD or Linux software packages, when they've been compiled for the platform. Compiled binaries are normally distributed as OS X Packages; but may still require command-line configuration or compilation. Companies such as Fink provide precompiled or preformatted packages for many standard packages.
Here, courtesy Apple Computer's press relations, is information on OS X:
"Mac OS X is a super-modern operating system that combines the power and stability of UNIX with the simplicity and elegance of the Macintosh. Featuring the stunning new user interface called Aqua. Mac OS X makes everything on the Mac even more intuitive for new users, while providing powerful, customizable tools for professionals. Aqua includes a number of innovative time-saving features—including a new Finder and the Dock—designed to help you navigate and organize your system, and give you instant access to your most frequently used applications, folders, and minimized windows. Aqua brings your desktop to life with expressive icons, vibrant color, and fluid motion. This new interface is built on three cutting-edge graphics technologies—Quartz, OpenGL, and QuickTime—bring graphics capabilities to Mac OS X beyond anything ever seen in a desktop operating system. But good looks are only the beginning. At the foundation of Mac OS X lies an industrial-strength, UNIX-based core operating system—called Darwin—that delivers unprecedented stability and performance. Darwin provides Mac OS X with powerful, advanced features such as protected memory, preemptive multitasking, advanced memory management, and symmetric multiprocessing—making your Macintosh more responsive, faster, and more reliable than ever before.
Great applications built-in. Built using many of the same technologies that power the Internet itself, Mac OS X gives you the power to get online faster and easier—and do more when you get there—with best-of-class Internet applications such as Mail, Microsoft Internet Explorer 5.1, QuickTime Player and iTools. Seamless device connectivity and industry-leading applications make Mac OS X the ultimate platform for your digital lifestyle. It includes powerful, easy-to-use tools for making your own movies, managing your music, and capturing photos from your digital camera. And with built-in support for burning music and data CDs, playing DVD movies, and even authoring your own DVDs, Mac OS X makes it easier than ever to share your creations with others. Get Started Today. Best of all, Mac OS X is simple to install and easy to learn. The Classic technology lets you run thousands of existing Mac OS 9-compatible applications, while the powerful features of Mac OS X provide a foundation for great new applications that are “Built for Mac OS X.” So whether at the office, at home, in the studio, or in the classroom, Mac OS X will make your computer run faster and more reliably, while bringing the simplicity and familiarity of today’s Macintosh to a whole new level. "
Obviously this is simply Apple's biased take on their new OS, but it is more than before-not a valid excuse for it lacking. I am consulting a more informed authority on the subject for use of their content.