Applying Copyleft To Non-Software Information
http://www.gnu.org/philosophy/nonsoftware-copyleft.html

Stutz, Michael
GNU

Abstract:

Copyleft contains the normal copyright statement, asserting ownership and identification of the author. However, it then gives away some of the other rights implicit in the normal copyright: it says that not only are you free to redistribute this work, but you are also free to change the work. However, you cannot claim to have written the original work, nor can you claim that these changes were created by someone else. Finally, all derivative works must also be placed under these terms.