What is Copyleft? 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 as well.
The simplest way to make a program free software is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. Intellectual Ventures Home. Intellectual Ventures’