Linux

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 for more information on Linux and BSD differences.