Browse Amazon Books: Berkeley Standard Distribution (BSD) - FreeBSD, NetBSD, OpenBSD, Darwin
When the Regents of the University of California got out of the *NIX business, they did a remarkable thing. They released the source code for BSD 4.2 to the public domain. (Considering the public investment in BSD through ARPA/DARPA, it was a reasonable decision -- but reasonableness seldom has much of a role in the assignment of intellectual property rights.)
Their benevolence resulted in FreeBSD, which in turn begat NetBSD, which in turn begat OpenBSD. Darwin came later, and I suppose it would be an outgrowth of NetBSD. Apple's OS X is their proprietary GUI running on Darwin for the Power PC. OpenDarwin is the open-source component of the whole OS X package.
The largest and oldest of the "free" BSDs - FreeBSD is the source from which the other more specialized BSDs are derived, and also the most tolerant of "newbies"
Some of the BSD crowd thought it would be nice if the OS ran on something besides IBM-compatible PCs. Net BSD has been ported to just about any platform you can think of.
OpenBSD emphasizes stability and security. Derived from Net BSD, OpenBSD lags a bit in features, but its code is exhaustively audited for bugs that could become vulnerabilities. It is the most secure OS available, period. Ideal for internet servers and firewalls.
Open Darwin is the Open soure version of the Apple OS X internals. The i386 version is something of a curiosity at present. You cannot simply run OS X on a IBM-compatible PC, the Apple GUI is a separate application.