According to their website, "Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools.
Our free, easy-to-use copyright licenses provide a simple, standardized way to give the public permission to share and use your creative work — on conditions of your choice. CC licenses let you easily change your copyright terms from the default of “all rights reserved” to “some rights reserved.”
Creative Commons-licensed materials are NOT always covered in citation manuals. We recommend that you cite the material as you normally would given its format, being sure to include the URL. Creative Commons licenses require that the creator(s) be cited. Do your best to find the name of the creator, not just a username or pseudonym. In addition, indicate the type of license and the URL of that license.