There are four main BSD variants. Three of these (FreeBSD, OpenBSD, and NetBSD) are totally free; the fourth (Mac OS X) is technically the core part of an operating system that most wouldn't even consider a BSD variant. To understand the differences between the various versions, let's briefly recap the history of BSD to understand how the different versions have developed.

Today's BSD variants are open source versions of the original AT&T Unix operating system. In fact, they all come from the Unix developed at the University of California Berkeley, and BSD is actually short for Berkeley Software Distribution. A significant part of the original BSD code was based on the AT&T Unix code, which wasn't free. Through efforts on the part of a few key members of the original BSD development team, such as William F. Jolitz, the final parts of the code were developed under an open source license and produced 386BSD.

In 1993, 386BSD was forked into two of the main versions we know today: NetBSD and FreeBSD. They were formed with different aims and goals. Not surprisingly, each has its own history. OpenBSD, the third variant, arrived in 1996 and was developed specifically to address some of the security concerns in the other variants.

The BSD incorporated into Mac OS X is known as Darwin. It is available as a completely separate component. Darwin itself is derived from the BSD layer of the NextStep operating system, developed by NeXT, the company set up by Steve Jobs after he left Apple in the 1980s. Technically, Mac OS X is based on the FreeBSD core, with OS X 10.3 based on FreeBSD 5.x. It is, however, extremely customized beyond the base BSD code. The key benefit with Mac OS X is the Aqua GUI that allows OS X to operate like the original Mac OS operating system but still have all the benefits and flexibility of an efficient BSD kernel.