GPL GNU General Public License
The Free Software Foundation's copyleft license. Richard Stallman, 1989. Requires derivative works to be distributed under GPL. Most famous GPL project: the Linux kernel (v2).
v1 (1989), v2 (1991, the Linux kernel stayed there), v3 (2007, added anti-Tivoization + patent grant). LGPL is the weakly-copyleft variant (linking from proprietary code OK). AGPL closes the SaaS loophole (must release source even if just hosting). Corporate battles: Apple swapped bash → zsh in macOS to avoid GPLv3. Linus Torvalds has said Linux will never upgrade to v3 ("v3 is wrong"). Apache 2.0 + MIT are more business-friendly alternatives.