GPL, AGPL, LGPL, MPL are popular open source licenses and they are all included in the category of copyleft licenses. Copyleft licenses specify that the same license terms must be maintained in derived works. This means that if you use code released under a copyleft license, distribution of your code must follow the same license terms in order for others to use it. Compared to permission licenses, which have no such restriction, copyleft licenses are considered restrictive, although they can guarantee open access to developers.

One of the most famous licenses in the copyleft group is the GNU General Public License, abbreviated GPL. Since it is a copyleft license, any software that uses all or part of the GPL code or references the GPL library must distribute all of its source code under the same license. This means that if you use some GPL libraries in your software, you must use the same license when distributing your software so that others can use your code for free. There are several versions of the GPL and version 3 is the latest. Compared to GPL v2, GPL v3 facilitates international use outside the United States and provides more clarity regarding patent licenses.

The Affero GPL, also called AGPL license, is very similar to the original GPL. The only difference between AGPL and GPL is that AGPL licenses consider network usage as distribution, thus closing the famous loophole of the GPL, the so-called Application Service Provider (ASP) loophole. Because of this loophole, software-as-a-service (SaaS) providers, since they have not technically distributed the GPL code in the traditional sense, can legally run that code or program over a network to make money while being exempt from the terms of the GPL license. AGPL licensees close this loophole by defining network usage as distribution as well.

The Lesser General Public License, known as the LGPL license, offers less restrictive terms compared to GPL licenses. GPL requires developers to release all derivative works regardless of use. In comparison, LGPL only requires authors to release works based on the LGPL-licensed components. If the author only uses the LGPL components, for example an LGPL library, he or she does not have to release it as open source. For example, if your software uses LGPL source code, you must release your code as well as the original GPL licenses. However, if you only use an LGPL library instead of the code snippets, you do not have to release your software as open source.

The Mozilla Public License, known as the MPL license, is compatible with GPL. It has less restrictive requirements than GPL and LGPL. GPL also requires the entire project to be open source as long as your project contains GPL code or links to a library. However, MPL licenses allow you to keep your own work closed source or proprietary as long as the MPL code you use is kept in separate files. In short, MPL licenses require the files containing MPL code to be kept open source while allowing you to keep your own files proprietary. Compared to the LGPL license which is based on a library-related rule, MPL is based on a file-related copyleft rule, which means that any file that does not contain MPL material can be kept closed source.

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