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 GNU project, our aim is to give all users the freedom to redistribute and change GNU software. If middlemen could strip off the freedom, we might have many users, but those users would not have freedom. So instead of putting GNU software in the public domain, we “copyleft” it. Copyleft says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.
Creative Commons aids you sharing your knowledge and creativity with the world while protecting your rights.