A separate family of operating systems. It's a direct decendant of the old UNIX. FreeBSD, OpenBSD and NetBSD are the most common distributions.

It shares a lot of the same userspace tools as Linux (the GNUserspace, if you will), so if you're used to the command line, a lot will seem familiar at a first glance. However, the underlying mechanisms can feel a bit strange at first once you try it out.

I used FreeBSD as my daily driver back in the day, but now I'm also on Mint, mostly because I used linux more at work.

I've been meaning to check out BSD these days to see how far it's come since I last used it 20ish years ago, I just haven't gotten around to it.

I also remember hearing someone say that there are two things from Berkley that messes with your head: BSD and LSD.