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Creative Commons is a nonprofit organization that works "to increase the amount of creativity (cultural, educational, and scientific content) in “the commons” — the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing" (http://creativecommons.org/about/what-is-cc).
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.
Check out the Creative Commons website for more information and details about CC licenses.
Here is a wiki on best practices for how to give attribution (credit) to images with CC licenses.