Since Linux (official Linux website is linux.org) is free and most of the software for Linux is free there has to be someone to put the operating system and the software packages together. That person continues to be the originator of Linux, Linus Tordvalds. There are several organizations that create Linux distributions - BUT - there is only 1 Linux, with numerous distributions.
There are about six main branches or families of Linux distributions. The six branches are the Debian/Ubuntu family, the Arch family, the Red Hat/Fedora family, the Slackware family, the Gentoo family and then there is the kernel source code family. While there are many distributions, the one thing that they have in common is that they all use a Linux kernel.
In addition to the Linux kernel, most distributions also use the GNU utilities. Because the GNU term in the distribution title is a political one, you will see some distributions listed as GNU/Linux and some with just Linux in the name.
You can find a lot of good information about most of the popular distributions at http://distrowatch.com/ - this is a good web site to bookmark
If you Google "list of Linux distros" you can find a plethora of information. Here is another good list: https://lwn.net/Distributions/
The KEY POINT to remember is - there is 1 and only 1 Linux, but there area LOT of different distributions. In this course we will focus on the Ubuntu distribution, version 18.04