In 1991, Linus Torvalds (then a college student in Finland) decided to learn about computer operating systems by trying to write one himself. He based his work on another popular operating system called UNIX. After getting some of the basic features working, Linus released his work on the Internet and invited other people to help. Many responded, and that was the beginning of a world-wide effort. The result is the operating system that has come to be known as Linux.
Linus released his operating system under the GNU Public License. This license permits anyone to copy, sell, give away, and even modify the operating system. The only restriction is that when you sell it or give it away, you have to make the source code available and you have to release any modified versions under the same license. Linux is "free" in the sense that no one can appropriate it or control it. You are free to do what you want with it, as long as you don't try to block anyone else's right to do the same.
Linux is also "free" in the sense that you don't have to pay for it. Since Linux is a large, complicated system, various companies and organizations have put together "Linux Distributions," and some charge for their service.
The distribution that we are using at HWS as of Fall 2016) is Linux Mint 18, with the Cinnamon Desktop. It is based on Ubuntu Linux, but has a different and more traditional user interface. Ubuntu is, in turn, baed on the Debian GNU/Linux distribution. Other popular distributions include Kbuntu (a spin-off of Ubuntu), OpenSuSE, Red Hat, and one of the very first distributions, Slackware.
(I should mention that Linus Torvalds is only responsible for the part of the operating system known as the "kernel". Properly speaking, the kernel is the operating system, but you can't use a computer that has no other software besides the kernel. You need a user interface and programs to run. You (or someone) also need tools for developing new programs. All these features are included in every Linux distribution. Most of them, like the kernel, are covered by the GNU Public License or something similar. Because the GNU project existed before Linux, some people prefer to refer to the total system as "GNU/Linux" rather than simply "Linux.")