Since the beginning of software development, we have seen the need for making sure that the software, programs, code and so on get sold, bought and distributed according to rules set by the owner(s). That’s where copyright came into play, allowing authors to protect their work from unauthorized copying or selling by third parties.
But what’s the case with open-source software? Is it under a copyright license, limiting everything that we know open-source for, or is it just uploaded code that anyone can do anything they please with? The answer lies in Copyleft, which is a great incentive for people to improve innovations.
In simple words, Copyleft is essentially copyright with different terms. It allows people to freely change and spread newer copies of a work without any legal issues. Most likely, it has the need to keep the state of the rights - meaning that when a free (as in freedom, not in monetary value) Copyleft-work was used to create a new project, the new project shall remain free to everyone as well.
But to be more specific, Copyleft licenses have three different grades:
No Copyleft - this type of license does not restrict any of the licensing on further copies/versions. It allows the developers to pretty much do anything with the work, but doesn’t give the end users any peace of mind that the further work will remain free and open-source.
A great example for this would be the BSD license, which imposes very minimal restrictions on the distribution of software.
Weak Copyleft - this means that the further derived versions might not have the license, as it depends on how it was derived. This is the method used with software libraries, as the software can be linked with the library and further distributed without the new software needing a license. Some part of the code may remain closed-source or be released under a different license.
An example of weak Copyleft would be The Mozilla Public License (MPL), which can be described as a hybrid of the modified BSD license and the GPL license. It later got ruled as GPL-incompatible, and was relicensed. They are still using it for the Firefox web support.
Strong Copyleft - this is the Copyleft license that creates the need for developers to make their program fully available when redistributed to the public. That includes any components or libraries that might have been used.
The most popular example is definitely the GNU General Public License (GPL) that guarantees end users the ability to study, share, run and modify the software under this license. Their third version was released in 2007 with the final version of the license text. Many popular works use this license, like the Linux kernel, Git, Wordpress, MySQL and Notepad++.
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