GPL or General Public License, sometimes also called GNU GPL, is the most commonly used free software license. It was written by Richard Stallman of Free Software Foundation for GNU Project. This license allows software to be freely used, modified, and redistributed by anyone. WordPress is also released under the GPL license, which means that WordPress is an open source software that can be used, modified, and extended by anyone.
The WordPress project considers GPL license to be its philosophical equivalent of the bill of rights. It provides the core basic values that the project believes in.
- Anyone can download and run the software
- Anyone can modify it
- Anyone can redistribute free copies of the software
- Anyone can distribute modified versions of the software.
One of the primary aspects of the GPL is that it uses something known as copyleft. Copyleft is a play on the word copyright, but the concept is very similar. Basically it uses copyright law to protect modified versions of a work but requires the same protections to be applied to those versions. For this reason any work based on WordPress inherits the GPL license. This has caused some controversy as some companies and individuals tried to sell WordPress Themes and Plugins under non-compliant licenses. However, there is a strong sentiment among the community to protect the WordPress bill of rights. The WordPress project co-founder Matt Mullenweg is very vocal about it.