UNIX, BSD, Linux systems
UNIX began with Ken Thompson who first thought of the idea of UNIX. Dennis Ritchie was the first collaborator. Ritchie created C, then used C to implement UNIX. Both previously worked on Multics, reused ideas from it. Douglas McIlroy introduced the idea of pipes, a major element of UNIX.
BSD is a descendant of the original Bell Labs source code, developed further by University of California, Berkeley. The Internet programming interface (TCP/IP sockets) originated on BSD.
Linux is a clone of the UNIX kernel, written from scratch. Most of the surrounding user-space software was developed by GNU.
Both BSD and GNU/Linux are powerful stable operating-systems. Arguably, BSD is more stable and faster than GNU/Linux (stable also means less changes to the OS). BSD's C library is simpler and faster than GNU glibc. Linux kernel is well-organized, modular, easy to modify, which led to the popularity of Linux. BSD kernel needs better organization, much code is arcane.
GNU/Linux is actually sssllloooooooowww. That GNU/Linux is slow is hidden by today's fast hardware. GNU/Linux was installed on a 68040 Amiga which ran Amiga OS 2.x fast, but the Amiga crawled trying to run GNU/Linux.