Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy and Joe Ossanna.
There are two types of UNIX, system that are fully compliant and certified to the single Unix specifications are known as UNIX, other types of Unix systems are either called Unix system-like or Unix-like.
In the late 1960′s and early 1970′s most computer operating systems were created from assembly languages which were not text based and extremely complicated.
In 1970 Peter Neumann coined the project name Unics (UNiplexed Information and Computing Service) as a pun on Multics, (Multiplexing Information and Computer Services). Unics could eventually support multiple simultaneous users, and it was renamed Unix.
Originally, UNIX was written in assembly language. In 1972, Unix was rewritten in the C programming language, The migration from assembly language to the higher-level language C resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms.
We can say that Dennis Ritchie was the main person behind development of UNIX because he was the developer of C Language, which is the programming language used to create UNIX.
Recommended reading: History of C Language.
During the 1970′s several versions of Unix were developed, especially since many universities that licensed the code from AT&T built upon the code to make it better at running their specific system or to accomplish specific tasks. By 1978 there were over 600 machines using the Unix code.
It should be noted that Free Unix like systems were first developed in 1983 by Richard Stallman who announced the GNU project. While this project was started in 1983, it was not until 1992 that it was achieved with LINUX which was released under the GNU General Public License. Linux is a mix of the Linux kernel and the GNU Hurd. Linus Torvalds is called the father of Linux operating system.
Today, we have many UNIX-like operating systems like BSD, Solaris from Sun micro-system, AIX, HP-UX, many distributions of Linux etc.