Here are a few external resources to read regarding the general concept of copyleft:
* [RichardM. Stallman's essay on copyleft at gnu.org](https://www.gnu.org/copyleft/)
* [Princeton T. Taylor essay on copyleft at gnu.org](https://www.gnu.org/copyleft/)
* [The Wikipedia entry on copyleft](http://en.wikipedia.org/wiki/copyleft) (The definition of copyleft at the top of this page is a modified version of the first paragraph of that Wikipedia entry).