"Creative Commons" is a tax-exempt charitable corporation that provides free tools that let authors, scientists, artists, and educators easily mark their creative work with the freedoms they want it to carry. You can use CC to change your copyright terms from "All Rights Reserved" to "Some Rights Reserved." Their licenses help you keep your copyright while inviting certain uses of your work — a "some rights reserved" copyright. (For example, this very paragraph has been taken from their Home and About pages.)
A great comic describing "How It Works" is available on the Creative Commons website here.
JoCopedia would like to thank Creative Commons for their part in bringing the music of Jonathan Coulton to the world.