system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The relationship between UNIX, LINUX, and Ubuntu

Very good responses, thanks.
gpt
Unix is an operating system that was originally created at Bell Labs in the late 1960s/early '70s. In the '70s and '80s it became very popular, and various derivative versions were made - some by commercial companies.

In the early 1990s, Linus Torvalds created Linux while he was at university in Helsinki. Linux isn't based on Unix, but is more like a clone or remake. So to the user it mostly behaves the same as Unix, although technically it isn't derived from Unix. Linus wanted there to be a free version of Unix that could be used on PCs (at the time most versions of Unix cost money and required expensive computers). It became very popular, other people contributed code to it and it is now one of the most-used operating systems in the world.

Linux itself is just a *kernel*. The kernel is the core of the operating system - it "sets the stage" on which everything else runs. The complete system is often referred to as "Linux", but it's actually Linux plus a whole load of different pieces of software from different projects.

There are many different Linux *distributions* (or *distros*). A distro takes all of these different components and assembles them into a complete system that you can install on your computer. Although the end result is usually largely the same, different distros do things in slightly different ways. Ubuntu is one example of a distro. Others would be Debian, Fedora, Gentoo, etc.