Best described by GNU “Copyleft is a general method for making a program (or other work) free (in the sense of freedom, not “zero price”), and requiring all modified and extended versions of the program to be free as well.” In general Copyleft ensures no middle man can convert the program into proprietary software, while still allowing anyone to change and redistribute the software.
It also provides an incentive for programmers to add to free software and helps those who want to contribute to free software get permission to do so.
Strong Copyleft demands that all the derived work must retain the license of the original. It cannot be made proprietary - this is the best option for end users but places some restrictions on developers and is difficult to integrate into into hybrid system that also use proprietary licenses.
Example: The GNU General Public License The GNU GPL is a series of free software licenses that guarantee end users the freedom to run, study, share, and modify the software. Written by Richard Stallman, the founder of the Free Software Foundation (FSF). Prominent programs licensed under GPL includes the Linux kernel.
Weak Copyleft also requires the retaining of the license but set some exceptions to better allow software integration with proprietary systems. This means that if you have a collection of programs that includes both free and proprietary software, the Copyleft doesn’t dictate the licensing of the new software.
Example: The Mozilla Public License (MPL) The MPL is a free and open-source software license developed by the Mozilla Foundation that seeks to balance the concerns of proprietary and open-source developers. It is used for many Mozilla projects like Mozilla Firefox & Thunderbird and has also been used by Adobe and LibreOffice.
No Copyleft do not limit the licensing of derivatives. The software is free but it is possible to derive proprietary applications out of it. This option gives the largest freedom to develipers. Example: The MIT (X11) license The MIT license puts only very limited restriction on reuse and is therefore highly compatible. It was found to be the most popular software license (in 2020) and is used by many popular projects like Ruby on Rails, Node.js, Lua & jQuery. Google, Facebook and Microsoft all use the MIT license.