In 1983, an American programmer named Richard Stallman came up with the idea of a open operating system which he called GNU (the acronym of which stood for “GNU’s Not UNIX.”) UNIX, at the time, was a popular operating system with a stable architecture, but it wasn’t open. What Richard Stallman was creating wasn’t just an operating system: it was also a social movement. By creating GNU and liberating it as copyleft (for free distribution – the opposite of copyright) under a GPL license (see below), a global community of developers was created who, to this day, share code and collaboratively develop systems. Years later after Richard Stallman showed his ideas through the internet, Linus Torvalds, who, in 1991 was an engineering student in Finland, created the LINUX nucleus with the collaboration of many programmers, which Stallman then published under a GPL license. Afterwards, GNU and LINUX were combined to create an open operating system that was completely functional. It’s typically known as "GNU/LINUX" or "LINUX distribution".
GPL (General Public License) was the first copyleft license: derived works could only be distributed under the same license. GPL is the most frequently-used license in the world and guarantees that the rights for end users allows them to use, share, study or modify software licensed under GPL. Companies or users who distribute their software under a GPL license can do so without charging users, but commercial use is also allowed, as well as using it as a proprietary software tool.