BSD is "Berkley System Distribution", a derivative of the original UNIX[1], while Linux as an OS is a combination of the GNU tools (written to be unix-ish) and the Linux kernel.
Comparing FreeBSD and Linux, the hardware support is roughly equal. The most notable difference might be that there's no 3d support for radeon cards in FreeBSD. [2]
This is purely my opinion: The BSDs tend to be cleaner and better documented, while many linux distros provide graphical tools, something the BSDs lack.
FreeBSD (and NetBSD, I think) can run Linux binaries with no (discernible) speed loss, and more or less all the software available for linux can be compiled on a BSD. This is automated by the ports system (pkgsrc in netBSD), a neat system for installing/managing software.
Personally, I keep coming back to FreeBSD because it works, does what I need, and I've grown to like the way it does things. It's a matter of taste, mostly.
As for slackware, I have no idea.
bountyhunter: Odd. Might of course not be their fault, but they ought to respond to mail.
[1] Long, messy story. There was a court case where AT&T sued BSDi for selling an OS that incorporated parts of their code, but when it came to what parts, it ended up being a few small files that were rewritten within a week or so. They settled out of court, and the only loss for BSDi was that they couldn't call their OS (the direct ancestor of Free/NetBSD) BSD UNIX anymore.
[2] Then again, the Linux drivers are not exactly good, either.
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