Copyleft

What is copyleft and its variants?

Copyleft is characteristic of some free licencies for software, art, music or other creations. Copyleft has different versions according to the distribution and freedom terms attached to it. The types are strong, weak and non-copyleft. Work under strong copyleft license may not be used as a part of a project or work with other type of license that is not compatible with the strong copyleft. Weak copyleft on the other hand, allows linked works be licensed under other types of licenses whereas the changes made to the work under weak copyleft must be licensed as weak copyleft. Non-copyleft free licenses allow producing proprietary work.

The best known real-life example of strong copyleft is GNU Public License. Weak copyleft licensed example is GNU Lesser General Public License and also Mozilla Public License. The first use of non-copyleft license was with Berkeley Software Distribution for the first free Unix. From there on the license is often named BSD-like free license. Under this category is also Apache License.