Darwin is a standalone, BSD-based operating system. (BSD, short for Berkeley Software Distribution, is a family of UNIX variants descended from Berkeley’s version of UNIX.)
Darwin is the core of Mac OS X. The Darwin kernel is based on FreeBSD and Mach 3.0 technologies and provides protected memory and pre-emptive multitasking. Darwin runs on PowerPC-based Macintosh computers and a version is also available for x86-compatible computers.