Creative Commons is a nonprofit organization that makes it easier to share, reuse, repurpose and remix creative material by providing free licenses to let creators give permission in advance for certain uses of their material.
Creative Commons licenses are legal tools that let people who own copyrights pre-approve certain uses of their works. They can be helpful for creators who want their works widely used, and they can be helpful for people who want to find things that are free to use with clear permissions. The licenses are not an alternative to copyright--they are an option for sharing works, and they rely on copyright.
There are six licenses offered by Creative Commons, listed from most open (CC BY) to least open (CC BY-NC-ND), indicating how much freedom others have in using your work. “CC” is for Creative Commons and “BY” is for attribution. A “CC BY” license has only requirement, which is to follow attribution exactly in the way the creator wants. CC BY allows use in all possible ways, even for commercial purposes. The 5 other licenses are “CC BY” plus additional requirement(s).
"Ordering of Creative Commons licenses from most to least permissive," by Stephan Kulla, creativecommons.org, under the Creative Commons Attribution-Share Alike 4.0 International.