Darwin is basically everything below the GUI in MacOSX. It’s an OperatingSystem on its own, I know, I have it running on a PowerMac 7300/180 directly to my left. Nice to work in a familiar UNIX!
DarwinOS is a descendant of OpenStep/NeXTSTEP, which in turn is a descendant of 4.3BSD and Mach 2.0. Some strains of 4.3BSD Lite and FreeBSD (4.4) have made their ways into it as well.
Check it out for yourself: http://www.levenez.com/unix/
The PureDarwin project aims to make PureDarwin easier to use by providing documentation and an installation and live ISO.