Short answers, pulled from the story.
Open-source software is released under a license granting users the rights to use, study, change, and distribute the software and its source code for any purpose. Proprietary software restricts access to source code, permits no modification by users, and requires fees for licensing, whereas open-source software imposes few or no such restrictions.
Christine Peterson suggested the term open source in 1998 at a meeting of free software supporters. That meeting led directly to the creation of the Open Source Initiative, which formalized a definition of open-source software based on the Debian Free Software Guidelines written primarily by Bruce Perens.
GNU is a free operating system Richard Stallman began in 1984, after resigning from MIT in response to proprietary software restricting the collaborative hacker culture in his lab. Its name stands for Gnu's Not Unix. In 1991, GNU was combined with the Linux kernel written by Linus Torvalds, and the resulting system is now commonly called Linux.
Permissive licenses such as MIT, BSD, and Apache allow recipients to distribute derivative works under any license they choose. Copyleft licenses such as the GNU General Public License require derivative works to use the same license, ensuring the software and its descendants remain open. The distinction was established in 2008 as legally enforceable following the Jacobson v Katzer ruling.
Eric S. Raymond described the two models in his 1997 essay The Cathedral and the Bazaar. The cathedral model is centralized development with defined roles; the bazaar model treats users as co-developers, favors early and frequent releases, and relies on Linus's law: given enough eyeballs, all bugs are shallow.
A 2024 estimate valued open-source software to firms at $8.8 trillion, with firms projected to spend 3.5 times more on software if they had to replace it with proprietary alternatives. A survey also found that 78% of companies report running all or part of their operations on free and open-source software.