"BSD" doesn't really exist as far as I'm aware. It was a proprietary operating system made at Berkeley for studying OS design.
OpenBSD/FreeBSD/NetBSD are open source continuations of the source-available 4.4-BSDlite code (removing AT&T proprietary extensions iirc).
OpenBSD follows BSD principles but focuses on code clarity and security.
FreeBSD tries to be very flexible, putting user-experience over security. (it has to be noted that OpenBSD is very usable, but lacks a lot of nice features like ZFS and DTrace that FreeBSD supports).
NetBSD is all about being incredibly lean and portable. NetBSD will run on basically anything, even things that Linux and other *BSD's have no hope of running on.
There absolutely was a real-world non-research operating system called just "BSD", and it ran primarily on DEC VAX hardware to start. Other Unixes like SunOS eventually started basing themselves on it. The last official Berkeley Software Distribution was 4.4BSD-Lite in 1994, which by then had been ported to many other platforms, including i386. The Computer Systems Research Group which maintained BSD disbanded the next year.
OpenBSD/FreeBSD/NetBSD are open source continuations of the source-available 4.4-BSDlite code (removing AT&T proprietary extensions iirc).
OpenBSD follows BSD principles but focuses on code clarity and security.
FreeBSD tries to be very flexible, putting user-experience over security. (it has to be noted that OpenBSD is very usable, but lacks a lot of nice features like ZFS and DTrace that FreeBSD supports).
NetBSD is all about being incredibly lean and portable. NetBSD will run on basically anything, even things that Linux and other *BSD's have no hope of running on.