Darwin is the "base" OperatingSystem that includes the Mach micro-Kernel and a BSD-compatible (mostly FreeBSD-derived) UserSpace – hence the claim that MacOSX is Unix. On top of this sits all the MacOSX graphics environment (Aqua) and the programming APIs Cocoa and the older Carbon. These APIs don't really use the Unix way of doing things – for example, MacOSX has its own "!NetInfo?" service for resolving things instead of using /etc/passwd or /etc/hosts.