Linux was developed by Linus Torvalds and introduced to the world in 1996. It was combined with the aforementioned GNU tools - a set of open source programs - to produce GNU/Linux, a usable operating system. This kind of configuration is what we call a "Linux distribution" (or a "distro").
Back in 1996 Linux had limited support for the hardware available in the day, but then there wasn't much to support. Today it supports a lot of hardware including mobile phones and tablets (in the form of Android.) It can be found on a very, very large range of computer hardware and is actually the operating system of choice for the world's top 500 super computers... all of them.
Fun fact: the #1 super computer in the world is in Japan and has 7,630,848 CPU cores. Most computers in 2022 have 4.
Since the inception of Linux we've seen hundreds of distributions being brought to light. I'm not going to list them all here, but here is the latest top four you should be concerned about (in no particular order):
- RedHat Enterprise Linux (RHEL)
These are the Linux distributions you're going to see in the wild in organisations across the world. These are what you might call "server side" distributions as they're not commonly found on the desktop. In fact in the case of Ubuntu there is, in fact, a desktop and a server version. With RHEL, "Fedora" is the desktop equivalent, as such. Debian you do find on both platforms. CentOS is usually server-side only.