What Are UNIX, Linux, and GNU? UNIX is a proprietary, command-line-based operating system originally developed by Dennis Ritchie and Ken Thompson (among others) at AT&T's Bell Labs in the late 1960s and early 1970s. UNIX is coded almost entirely in the C programming language (also invented by Ritchie) and was originally intended to be used as a portable and convenient OS for programmers and researchers. As a result of a long and complicated legal history https://arstechnica.com/tech-policy/2011/07/should-we-thank-for-feds-for-the-success-of-unix involving AT&T, Bell Labs, and the federal government, UNIX and UNIX-like operating systems grew in popularity, as did Thompson's influential philosophy of a modular, minimalist approach to software design.
During this period, Richard Stallman launched the GNU Project https://www.gnu.org/gnu/initial-announcement.html with the goal of creating "an operating system that is free software." GNU, confusingly, stands for "GNU's Not UNIX." This project is responsible for the UNIX-like GNU OS. Stallman also launched the related Free Software Foundation (FSF) on the principle that "any user can study the source code, modify it, and share the program" for any participating software.
I'll go deeper into what makes up an operating system in a minute, but the plot thickened when, essentially, the development of a very important low-level component called the kernel or GNU Hurd did not fully materialize. This is where Linux, a kernel developed by Linus Torvalds among others, entered the picture. According to GNU https://www.gnu.org/gnu/linux-and-gnu.html:
"Linux is the kernel: the program in the system that allocates the machine's resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system."
GNU purists argue that references to Linux as the complete operating system that exists today should instead be written as GNU/Linux https://www.gnu.org/gnu/gnu-linux-faq.html, in acknowledgment of the pair's symbiotic relationship. Others tend to focus on the fact that Linux (with no prefix) has become a more mainstream term and the logic behind the GNU/Linux nomenclature could expand ad nauseam to GNU/Linux/Windowing System Name/Desktop Environment Name/Etc. For the purpose of this guide, I'll use GNU/Linux.
Other UNIX-like operating system options exist too, notably FreeBSD and Qubes OS, which work with their own kernels and software. The histories of these projects could fill many books, but this brief summation should be enough to contextualize some terms you may come across.
More @ https://in.pcmag.com/windows-10/134555/what-is-gnulinux