Uni-verse
The history of the UNIX family is definitely long enough to confuse beginners. What is Linux? Is GNU/Linux even an operating system? BSD? Fedora? Red Hat? What are they? Is Linux Unix? Was there really an operating system called UNIX?
Just like how people would usually study royal families, here I am trying to draw myself a family tree for the Unix family (link) to better understand the relationship between two operating system.
The family tree above has intentionally left out most systems created after Linux, that is, most Linux distributions, since there are simply too many of them to be listed here.
Unlike proprietary operating systems such as BSD (versions before 1991) and UNIX, which were usually distributed as a whole, that is, their proprietary kernels plus their proprietary programs, since Linux is just a kernel under the GNU General Public License and thus separates implementation and distribution, anyone can freely modify and distribute the kernel along with other programs and features.
Thanks to the GNU project (or the GNU system), distributors can add GNU programs to their distribution in a similar manner, and actually, most Linux distributions will use at least some GNU programs so that people always refer to Linux distributions as GNU/Linux distributions.
(Of course, you can avoid the GNU/Linux bundle if you want to, a notable example of Non-GNU Linux is Android, and even if you use a GNU/Linux bundle, there are more free programs in your system than just GNU programs. Firefox, for example, is a free program released under its own Mozilla Public License)
Anyway, there are numerous Linux distributions since they are all just combinations of free or non-free software (permitted by GPL and many other open source licenses) and a distribution can be even built upon another distribution (Ubuntu is a derivative of Debian and Manjaro a derivative of Arch Linux, just like macOS is a derivative of FreeBSD). What makes a distribution unique is the features provided by its distributor. Many distributions are backed by commercial companies such Red Hat (Fedora, CentOS and the already discontinued Red Hat Linux), Canonical (Ubuntu), SUSE (OpenSUSE). To make things easier, you can have a list of all major GNU Linux operating systems here.
To answer the questions asked at the beginning, UNIX, developed by AT&T, was once a proprietary operating system in its own right but now only represents a standard and all conforming operating systems, if certified, can call themselves UNIX. Linux is not a certified UNIX system, Apple’s macOS is. BSD, as a direct descendant of UNIX, was also a proprietary operating system before it was rewritten to remove lines and files owned by AT&T; while its development has stopped since 1995, many BSD derivatives (including Apple’s macOS) are still widely used today. The GNU project, initiated by Stallman, has created an ecosystem of free software and even more free software have been developed under the GPL and Linux is one of them. Since Linux is itself a free program, people can create any number of operating systems based on the kernel and these operating systems are called Linux distributions. Naturally, most distributors would also use GNU programs when they build their own distributions and such a bundle is called GNU/Linus. Fedora, Ubuntu, Arch, Debian are all Linux distributions that use such a bundle and can therefore also be called GNU/Linux distributions. In the meanwhile, distributors don’t always start from scratch and a distribution can be built upon another distributions, e.g. Ubuntu is built upon Debian, and then you may call Ubuntu Linux-based, GNU/Linus-based, or Debian-based. Finally, Red Hat is the company behind many Linux distributions, including Red Hat Linux and Red Hat Enterprise Linux.
You can find a more detailed (and more complicated) family tree of the UNIX family here.
