Questions about the command line and other Unix-like aspects of Mac OS X, including the Mach/BSD-based kernel and FreeBSD-based userland. You may want to consider using Ask Different instead.
This tag covers questions about the various Unix-like aspects of Apple's Mac OS X—more or less, the command-line interface. If your question is instead about things that aren't Unix-like (including most all of the GUI), that is covered another Stack Exchange site, Ask Different.
See also darwin if you're using/asking about the Darwin open source project.
For general information, Wikipedia has a Mac OS X article.