In its infancy AT&T Bell Laboratories permitted Berkeley and other universities to share the source code to their UNIX operating system. Berkeley used their software as a research base for a variety of investigations into operating system design throughout the 70's and 80's. Eventually the sum total of the systems that Berkeley students had developed from scratch for their research had replaced essentially every component of the original UNIX kernel, and in the early 90's the full Berkeley source code was released publically with a very generous license called the BSD License.
BSD pioneered many of the advances of modern computing. Berkeley's Unix was the first to include library support for the Internet protocol stacks, Berkeley sockets. By integrating sockets with the UNIX operating system file descriptors, users of their library found it almost as easy to read and write data across the network, as it was to put data on a disk. The AT&T laboratory eventually released their own STREAMS library which incorporated much of the same functionality in a software stack with better architectural layers, but the already widely distributed sockets library, together with the unfortunate omission of a function call for polling a set of open sockets (an equivalent of the select call in the Berkeley library), made it difficult to justify porting applications to the new API.
Like AT&T Unix, the BSD kernel is a monolithic kernel, meaning that device drivers in the kernel run in ring 0, the core of the operating system. Early versions of BSD were used to form Sun Microsystems' SunOS, founding the first wave of popular Unix workstations.