The GPL license was created by the free software foundation. The majority of free software is open source. However open source software with a restrictive license that prohibits derivatives is not published under the GPL. The GPL grants you the ability to copy, modify and distribute software. So for example if you modify a WordPress theme or plugin that is published under the GPL you have to ensure that the version you create remains free.
The Meaning of ‘Free'
Free in relation to the GPL is as in free speech not as in free beer. Technically this means that the GPL license has been created to defend our rights. If for example I create for you a WordPress plugin I have the right to charge for this. At the same time however if you decide to modify or change software that I have sold you there is a requirement for you to work under the GPL license that the software was initially published under. This means that the software has to remain free. Effectively meaning that further innovations need to be released back to myself and also where WordPress is concerned back into the community.
GPL and Copyleft
The best way to keep software free is to release it without copyright into the public domain. However this means that ‘middle men/women' can pick the software up, copyright, restrict and remove any freedoms that were initially associated with that software. The copyleft aspect of the GPL means that any software released under the GPL has to remain free software and cannot become the sole propriety of any individual.
WordPress vs Wix
There is an interesting case at the moment where Wix have been accused by WordPress of integrating WordPress software into a mobile app. This is all good and well from the point of view of the GPL, however at the same time Wix need to make their apps source code publicly available. There is more on Matt Wallenberg's blog. In order to satisfy the GPL Wix need to release the source code for their development back into the community. What this highlights is that if any application contains a segment of GPL code. Could be one line or a thousand, the entire application has to be released under the GPL
Why did I Choose The GPL?
All the WordPress plugins and themes I have developed or I am developing contain snippets of what can be called ‘WordPress Source Code' . I am free to develop with this code and resell it, however at the same time I need to ensure that the software and its customisation stays within the community. To ensure this happens I am in the process of committing all my plugins and what will be my first theme (when it is complete) to Github. This as far as I understand satisfies without question my commitment to the GPL. It will also protect me from clients who believe they can restrict my freedom through copywriting my work after I have sold it to them. This has happened in the past, and is the reason I ended up working with WordPress.
Peace of Mind
If at the outset of a business relationship clients are made aware of the GPL, my freedom to develop and acquire an income through software development is protected. Another important factor is that if work is conducted in this manner it benefits the greater good. My work with software licensed under the GPL will all soon be available on a Github page for Premier Web Design, I have numberous plugins, shortcodes and eventually at the end of this project a theme to introduce. All of these came into existence because of WordPress and its code: GNU General Public License Version 3