The immediate, obvious difference between Unix and Mac OS 9 is the user interface. Until Mac OS X, the various graphical interfaces available to Unix users all fell short of the elegance and polish to which Macintosh users are accustomed.
It is a tribute to the architecture of Mac OS X that you can use it as a Macintosh operating system and never have to see any significant Unix underpinnings. Never, that is, unless you want to learn Unix.
From a more technical point of view, when Mac OS X is compared with Mac OS 9 (and earlier Macintosh operating systems), several important differences stand out. As we noted earlier, Unix uses protected memory and preemptive multitasking, and has other capabilities that let applications share memory, processors, and applications in a stable and reliable way.
As a result, it is hard for one misbehaving application to affect any other application or the operating system itself. Yes, applications can still crash in Mac OS X, but rarely do they take the whole OS down with them. Not only will you suffer fewer crashes, but they'll impact your other work less.
Unix, and thus Mac OS X, is also a multiuser operating system. It's designed from the ground up with the assumption that many people will be using the computer, often simultaneously . Just as the applications' activities are kept separate, so too are the actions of each user. Even if 50 people are using the Macintosh, it is hard for any one of them to mess up the work of the other ones.
Another way Mac OS 9 differs from Mac OS X is in the arrangement of files and folders (called directories in Unix) and the way information about each file is stored.
You will also see an icon that looks like a house show up as a shortcut in the Finder navigation dialog, the Save dialog, and so on. This icon will be labeled with your short user name and represents your home directory . This concept of each user's home directory as the only place where you normally create files is a thoroughly Unix idea arising directly from Unix's nature as a multiuser system, and is a major change from Mac OS 9.
Apple has strived to mask these differences in Mac OS X, but they are still there, and you must be aware of them if you want to do serious Unix work (or even just be a Mac OS X "power user").
In your day-to-day use of Mac OS X, you'll find that many of the ways it differs from Mac OS 9 have more to do with Aqua, than with Unix. For example, the Dock was new to Mac OS X, but it wasn't a "Unix change." In this book, we'll focus specifically on the Unix characteristics of Mac OS X, not the differences that came from Aqua.