Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools.
Their free, easy-to-use copyright licenses provide a simple, standardized way to give the public permission to share and use creative work — on conditions of the author's choice. CC licenses let you easily change your copyright terms from the default of “all rights reserved” to “some rights reserved.”
Creative Commons licenses are not an alternative to copyright. They work alongside copyright and enable you to modify your copyright terms to best suit your needs.
Creative Commons offers a variety of different licenses ranging from Public Domain (the most open license) to licenses that require the work to be attibuted to the original author, for non-commercial use only, and doesn't allow any modification or adaptation to the original work (most restrictive license available).
The following infographic summarizes the types of licenses and their noted restrictions. Click the image to view the full infographic.
"How To Attribute Creative Commons Photos" by Foter under CC by SA