Just like Linux, BSD operating systems come in many flavours:
FreeBSD: an alternative to Linux for desktops and servers alike
NetBSD: ultra portable, it runs on almost anything that has a CPU made since the eighties (and beyond!)
OpenBSD: secure by default. Includes numerous built-in security features. Constantly developing new security tools
Open Darwin: the open source code behind Apple's latest OSX range
Apple Mac OSX: user friendly, the only non-free operating system featured here, requires Apple hardware