This is a quick overview of FreeBSD -- the OS, the community, the philosophy -- as I see it. Last update: 21 February 2008.
First and foremost, FreeBSD is a flavor of BSD Unix.
Back in the 1960s, some very smart people at AT&T Bell Labs invented an operating system. They called it UNIX, at least in part as a play on MULTICS, an older operating system. It has advanced and expanded over the years. It was originally pretty much thrust into the wild, open for anyone to play with who could afford the hardware. It ended up in the hands of the University of California at Berkeley.
At UCB, it got a whole lot of attention, and a whole lot of new code was written for it. Eventually, a graduate student named Bill Joy invented a text processor/editor called vi for the OS, then packaged together all the new code written at Berkeley with the core code into a single distributable OS package. It became the Berkeley Software Distribution of Unix. As the development of BSD Unix continued, a bunch of the new code was sent back upstream to be incorporated into AT&T UNIX and its other derivatives.
Eventually, AT&T gained the legal "right" to actually make money off UNIX, so commercial versions of the operating system started appearing. Later, AT&T tried to sue distributors of BSD Unix. As part of the fix to get the power-mad corporation off their backs, they ripped out what little AT&T code was left, and replaced it with original code. BSD Unix no longer had a direct genetic link to its parents, but the concepts were still all there.
Because AT&T, followed by its successors (since it eventually sold off the rights to UNIX), maintained strict control over mainstream UNIX development, it all basically fell into a single family of Unix, contrasted with the BSD Unix family that would eventually arise. The general design of the system ultimately reached a form known as System V. Thanks to the tight commercial control over the SysV line of development, any derivatives were pretty much unavoidably commercially distributed systems as well. By contrast, the BSD family that would develop was not fettered in the same way.
UNIX is a trademark, owned by The Open Group (whose name is somewhat misleading), which allows operating system distributors to use the UNIX trademark if their OSes meet certain criteria, and if those distributors pay The Open Group some hefty fees. For reasons of this trademark difficulty, I (and others) refer to Unix systems directly descended from the original UNIX in general as Unix, and systems that have been certified Single UNIX Specification compliant by The Open Group as UNIX.
Thus, there are the two major families of Unix: BSD Unix and SysV Unix. The code they share in common is almost entirely code developed on the BSD side of things. They are all, however, very much descendants of the original, "traditional" UNIX. This is in contrast to systems like MINIX and Linux, which were just created from scratch later specifically to emulate the functionality of the original UNIX to some degree on a less-powerful hardware platform.
Once the Intel x86 PC platform advanced to the level of sophistication and power of the i386 processor, it had finally reached a point where a feature-complete Unix system could actually run on it. This was when an effort to port BSD Unix to the x86 architecture produced 386BSD. 386BSD didn't last long, but two derivatives of it that did last were FreeBSD and NetBSD. Both are still being developed today, along with the other major free BSD Unix system, OpenBSD -- which was a fork of NetBSD.
Of the three major BSD variants, FreeBSD is probably the most easily suited to general "desktop" computer use, as most people are familiar with it. It probably has the widest range of multimedia capabilities, and definitely has the most extensive archives of application software available for quick and easy installation. It also has the largest user community of the three.
FreeBSD's motto is "The Power To Serve". Despite its strong "desktop" system capabilities, it is still developed primarily as a server OS. Many consider this a significant part of the reason it is such an excellent "desktop" OS: the development focus on suitability for server use ensures it is a secure, stable, easily managed operating system. These characteristics tend to be shared by the free BSD Unix systems to varying degrees to significant advantage over other operating systems.
These comparisons depend substantially on my own various levels of experience with the operating systems involved. As of this writing, only one comparison has been written, but others should be forthcoming, in time.