7. Copyleft licenses

 Copyleft allows creators to make their works free to be modified and improved by other people. Anyone who has their hands on that product can freely modify it and publish. However, it does not mean that copylefted work is not copyrighted. It uses the copyright to ensure that all modified works are guaranteed to be as free as the original work. That way no one can make the modified version into a proprietary work and strip the ability to freely modify it by other people. This ensures that all versions of the original work have the same original intellectual property rules.
 The main benefit of using copyleft is for example open-sourced software that can be improved by anyone. This greatly increases the potential of that software to be developed into something great.

Types of Copyleft Licenses

 GNU General Public License (GPL) is a popular example of a copyleft license for software and other kinds of work. It allows the work to be freely changed and published but doesn't allow it to become proprietary. GNU C++ compiler is a work made possible thanks to GPL license allowing people to contribute to developing it.
 GNU Lesser General Public License (LGPL) is similar to GPL but it allows for example a library under this license to be used by a non-GPL licensed program, even if it's a proprietary software.
 GNU Free Documentation License is another license but it covers functional documents and manuals. Because free software needs free documentation as well.

 

Sources:

https://www.gnu.org/copyleft/
https://www.gnu.org/licenses/lgpl-3.0.html
https://www.gnu.org/licenses/why-not-lgpl.html
https://www.gnu.org/licenses/gpl-3.0.html
https://www.gnu.org/licenses/fdl-1.3.html