There are several versions of Unix. Some past and present commercial versions include Solaris, AIX, and HP/UX. Freely available versions include Linux, NetBSD, OpenBSD, and FreeBSD. Darwin, the free Unix underneath Mac OS X, was built by grafting an advanced version called Mach onto BSD, with a light sprinkling of Apple magic for the windowing system.
Although graphical user interfaces (GUIs) and advanced features differ among Unix systems, you should be able to use much of what you learn from this introductory handbook on any system. Don't worry too much about what's from what version of Unix. Just as English borrows words from French, German, Japanese, Italian, and even Hebrew, Mac OS X Unix borrows commands from many different versions of Unix, and you can use them all without paying attention to their origins.
From time to time, we do explain features of Unix on other systems. Knowing the differences can help you if you ever want to use another type of Unix system. When we write "Unix" in this book, we mean "Unix and its versions," unless we specifically mention a particular version.