Copyleft licensing and its usage

Copyleft is ment for supporting the copyright laws by removing the copyright restrictions from the work for distributing and modifing the work with the requirement to retain the same rights for the works are created from the original work.

It is meant for modifying copyrights for works as music, art, documents and computer software.
Basically autor of the work can prohibit other people by reproducing, adapting, or distributing copies of his work, but author can give permission to distribute the copies of the work to reproduce the work with the same copyleft licensing scheme. With computer software, example is open source copyleft license which requires to show information about creator of the software and include the the source code.

Using copyleft person can codify his work with the license. Licensing can be either none, weak or strong. Many free software licenses are none copyleft licenses and are not requireing licensee to distribute the software under the same license, and makeing it widely possible to use in proprietary software. For example if you choose a none copyleft license for your software project, your code will possibly spread very quickly if its good enough. Many companies will be interested in it, if they can integrate this to their software. What you get is recognition and maybe become a new standard but, companies who are using you code are getting the profit. Good examples are Apache webserver license and X11.

Weak copyleft like LGPL is mostly used for creating software libraries, because it allowes other software to link to the library and redistribute it without legal issues. For example if you software project is using a weak copyleft license, then possibly your software is adopted by other software companies, because the license allows them to use your software together with theirs. It also means that they can make revenue from the code they develop on the codebase you are giving away free. Good example from weak copyleft licensing is C standard library.

Strong copyleft license such as GNU GPL means that all source code modifications, additions, or derivatives must use the same GPL license. For example if your software uses a strong copyleft license, then companies who are wanting to earn profit from your sourecode don't want to use your code as their connected to your original code, because there could be many conflicts with GPL license. This is the main reason why many companies are not allowing to use GPL in their projects.