What Is Darwin (and How it Powers Mac OS X)
"Darwin is the Unix-derived core that provides the underlying foundation for Mac OS X. At Darwin's heart is the XNU kernel; a Mach 3.0-based microkernel that has been modified to include portions of FreeBSD for performance reasons. Let's take a trip down into the core of OS X to learn more about the foundation that gives us one of the best user experiences in computing."