MAC OS X REPRESENTS A UNION OF many different operating system technologies. At its core you will find Unix, an operating system once reserved for high-end servers and workstations, now found on common desktop systems. With Unix comes a whole history of tools, computer languages, and runtime environments. At a higher level, you will find Cocoa, derived from the application toolkits found on NeXTSTEP and OpenStep — a result of Apple's merger with NeXT in 1997. In Mac OS X you will also find Carbon, a library made from elements of the original Macintosh operating system. The original Mac OS API remains for older projects, although Apple is de-emphasizing its role in Mac OS X. Other technologies have found their way into Mac OS X through the open source community, and Apple is hard at work developing new technologies unique to Mac OS X.
Although the operating system is composed of all these separate pieces, Mac OS X still looks and feels like a single piece of software. The Macintosh's legendary user interface still shines brightly today, providing a consistent look and feel for the applications on your system. When you sit down to use your computer, it just works for you. And aside from a few cosmetic differences — say using a command-line interface or a graphical interface — rarely are you aware of the differences between all these operating system technologies.
Under the covers, there are fundamental differences between these operating system technologies. For example, ...