A Unix-like operating system. Many distributions exist; the closest analogue to FreeBSD being the Gentoo distribution: It has a ports system and places an emphasis on building packages and the system itself from source.
FreeBSD has a Linux compatibility layer that allows it to run Linux binaries (ELF) as if they were native FreeBSD programs; this may be useful if you find yourself needing to run a program that's only available as a linux binary. This is rare, but it happens.
See this excellent article by Dru Lavigne at ONLamp or this other article (also by Dru Lavigne at ONLamp) for more information on Linux and BSD differences. Matt Fuller has also written a good introduction to FreeBSD for Linux users which is available here.