The command line and other Unix-like aspects of Apple's macOS (Mac OS X)
Apple's macOS (formerly known as Mac OS X) is a Unix variant built on a Mach/BSD-based kernel and FreeBSD-based userland. On this site, we cover the Unix-like aspects of the system, including the command-line interface, the filesystem, etc. Question about things that aren't Unix-like (including most of the graphical user interface) are covered at our sister site Ask Different.
See also darwin if you're using/asking about the Darwin open source project.