Copyleft comes from copyright which different types describe the distribution and modification of works like computer software, document, music, art.
According to GNU official webpage Copyleft is a general method for making a program(or other work) free, and requiring all modified and extended versions of the program to be free.
Simplest way to make program free is to name it uncopyrighted. It allows users of the program to share it, make improvements and by making changes distribute as proprietary software. People who receive the program have the freedom what the last developer has made. E.g Ogg Vorbis open source audio format specification and software implementation for lossy audio compression. Other non-copyleft license users are Apache, BSD licenses.
Weak copyleft refers to license where not all derived work inherit the copyleft license. Its inherit depends on how it was derived.Weak copyleft licenses are mostly used for software libraries by allowing links to other libraries. For example Mozilla Public License uses weak software license.
Strong copyleft provisions can be imposed to all derived works which means that the first creator of the works has the most rights. One of the free software licenses that use strong copyleft is the GNU General Public License. Other strong license is Design Science License that can apply to art, music, sports photography and video.
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