Definition
The GPL (GNU General Public License) is a copyleft license published by the Free Software Foundation: anyone may run, study, modify and redistribute the software, but redistribution must be licensed under the same terms and accompanied by the corresponding source code.
How it works
The GPL writes “modified and redistributed versions must stay open” directly into its grant, and copyright law enforces this so-called viral clause: once a distributed product includes GPL code, the whole must be offered under the GPL. The versions in common use are GPLv2 and GPLv3, and the Linux kernel is licensed under GPLv2.
Usage
Prominent software such as the Linux kernel, GCC and WordPress ships under the GPL. Companies using it need to watch the compliance boundary: internal use is generally fine, but distributing a derivative product to third parties triggers the open source obligations.
Related and confusable terms
fork describes the practice of copying a project and evolving it independently, while the GPL governs the licensing conditions under which a fork may be redistributed; permissive licenses such as MIT and Apache-2.0 do not require derivatives to adopt the same license.