Backstory: Richard Stallman began the free software movement in 1983 with the GNU project. “Free” software is defined as that which preserves its users’ freedoms, including the freedom to use, modify and redistribute the program as the user sees fit. The open source initiative, on the other hand, grew out of the free software movement in 1998 as a way to encourage open and collaborative software development. While the two movements have much in common, they are not identical, as all free software is open source but, depending on the license used, not all open source software is free software. To Stallman, the free software movement is a philosophy, while open source is a development methodology. The difference, he says, is important and words matter, so equating them by using the terms interchangeably is something which drives him batty.
Quotes: “I never imagined that the Free Software Movement would spawn a watered-down alternative, the Open Source Movement, which would become so well-known that people would ask me questions about 'open source' thinking that I work under that banner.” May 2000
“Calling freedom-respecting programs ‘open source’ leads people to ask, timidly, ‘Please, sir, may I have some more leeway,’ rather than saying they deserve freedom and mean to get it.” February 2015
“What we achieve by distinguishing free software from open source is to call attention to issues of freedom.” October 2007
“... I never advocated ‘open source’; that's the slogan of people who disagree with me. I advocate ‘free software’, free as in freedom.” July 2013
“... I am not a supporter of ‘open source software’ and never was….” December 2012