Unix is a multitasking operating system developed at Bell Labs around 1970 as an interal tool. Unix is written primarily in the C programming language, making it extremely flexible for running on multiple platforms. A large part of it's appeal is it's modularity, which is explained in the Unix Philosophy, an approach to making minimal and modular software that works effectively when combined.
After being released to the public a few years later, various Unix derivitives would be released, most notably being BSD (or 'Berkeley Software Distribution') from the University of California, Berkeley.