Creative Commons licenses are public copyright licenses that have been released for free by the Creative Commons non-profit organization. The licenses are free to use and they allow copyright owners to make their work available on a “some rights reserved” basis. Copyright owners can choose a license based on these factors:

• Whether attribution is required (if no attribution is required then the photographer may wish to release the photograph into the public domain instead of using a Creative Commons license);
• Whether derivative works may be made, and if so, whether the derivative works must be released under a similar license; and
• Whether commercial use is permitted.

A Creative Commons license has three “layers”: the legal language of the license; the human-readable summary of what the license does; and the machine-readable code that allows search engines and other computer tools to easily find images with a certain license. More than 1 billion works have been licensed under Creative Commons licenses, and the licenses have been enforced by courts in several countries.