What is it? Well it's like Linux + :-)
FreeBSD is a BSD based operating system that ran into legal problems similar to the ones that Linux is now finding itself in.
This slowed development down, letting a Certain Finn come up with a certain OS.
FreeBSD tends to be used on more servers than workstations and runs some of the internet's larger and more reliable web sites.
http://www.freebsd.org is the home page for it.
You might also like to try GhostBSD, based on FreeBSD, but as a live cd/dvd.