The Linux kernel, which lies at the heart of every Linux OS, was invented by Linus Torvalds a Finnish computer scientist, in 1991. However, Linux is open source, which means that anyone who wants to can edit it and publish their own version, which many, many people have, so I really need to talk about the two versions I have used, and compare and contrast them.

Red Hat Fedora

Red Hat Linux has been making what they call Red Hat Enterprise Linux, which is Linux, but you pay for it, and they supposedly give you tech support. To get you interested, they also have a free version, Fedora, which comes with no community support and lots of ads for Enterprise. Corporate socialism, if you will. It doesn't work.

Ubuntu

At the other end of the spectrum is Ubuntu, Linux for human beings.  Ubuntu was developed with the sole purpose of making Linux free and easy to install or double boot (i.e. to choose when starting the computer whether to use Windows or Linux) and it has been amazingly successful, bringing many people, including myself, into the Linux world.

Gnome and KDE

I'll keep this short: unlike Windows (and possibly Mac, I have no idea), Linux is text-based. In Windows, the lowest you can go is the windowing system. The disadvantage is that the windowing system cannot be changed without reinstalling the operating system. Linux, however has a program which handles all the graphical stuff like windows and screensavers and cursors. The two most common Linux windowing systems are Gnome and KDE, but there are many others. Although they look like different OS's on the outside, I could uninstall Gnome and install, say, Blackbox, without affecting my files or even my programs.