BSD (Berkeley Software Distribution), is derived from the UNIX operating system, and has been under development in one form or another since 1977. It is significant not only as a progenitor of UNIX-like operating systems in general, but also of the BSD Software license. This license has been used to distribute all manner of free, Open Source Software to this day.
BSD itself has long since forked into various projects with different individual goals, but the same general philosophy. Each distribution aims to make a stable and robust free operating system that is useful to the largest possible user demographic.