BSD family of Unix systems is based upon the source code of real Unix developed in Bell Labs, which was later purchased by the University of California - "Berkeley Software Distribution". The contemporary BSD systems stand on the source code that was released in the beginning of 1990's.
BSD is behind the philosophy of TCP / IP-network and the internet it is a developed Unix system with advanced features. With the exception of their own BSD / OS, the development of which has been discontinued, are currently four BSD systems available: FreeBSD, NetBSD, OpenBSD and Mac OS X, which is derived from FreeBSD.
There are also various forks of these, like PC-BSD - FreeBSD clone, or MirOS, a clone of OpenBSD. The purpose of such plugs to include various features not found in the aforementioned systems BSD, where these (forks) regardless of how well they are only strongly dependent. PC-BSD, for example, has more features than the graphical FreeBSD, but there were no significant differences between the two. PC-BSD can not breathe without FreeBSD; FreeBSD or OpenBSD are independent of each other.
The differences between Linux and BSD
1) BSD license allows users/companies to modify a program's source code and not to release changes to the public. In other words, BSD licenses allow commercial use and incorporation of a code into proprietary commercial products. This is how Microsoft incorporated BSD networking into their products and how Mac OS X earns money through muscles of FreeBSD.
2) BSD has the so-called "core system". The core of the system consists of basic utilities, and that beyond that, strictly considered as a supplement. Linux (not only the nucleus, of course), usually packaged as an entire system, where the difference is not visible.
3) BSD systems have also their stable version. With FreeBSD, for example, you have a FreeBSD-Release, FreeBSD-Stable, and a development version - Current, which is not stable and not recommended for a regular use. Some Linux distributions started to imitate this philosophy, but with BSD systems this way of making distributions has become a rule.