Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools. Creative Commons licenses allow material to be shared and reused under terms that are flexible and legally sound. If you're looking for content that you can freely and legally use, there are hundreds of millions of works, of all formats and genres, that are available to the public for free and legal use under the terms of a Creative Commons license.
Attribution You may distribute, remix, tweak, and build upon the work, even commercially, as long as you credit the original creator.
Attribution-ShareAlike You may remix, tweak, and build upon the work, even for commercial purposes, as long as you credit the original creator and license your new creation under an identical license.
Attribution-NoDerivs You may redistribute, commercially or non-commercially, as long as the work is passed along unchanged and in whole, with credit to the original creator. You cannot change the work in any way.
Attribution-NonCommercial You may remix, tweak, and build upon the work, as long as you credit the original creator and use the creation for non-commercial purposes.
Attribution-NonCommercial-ShareAlike You may remix, tweak, and build upon the work non-commercially, as long as you credit the original creator and license your new creation under an identical license.
Attribution-NonCommercial-NoDerivs You may share the work with others, but you cannot change it in any way or use it commercially. You must credit the original creator.
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So you've decided to use a work that contains a CC license?
One condition of all CC licenses is attribution. You can find some good (and not so good) examples of attribution here: https://wiki.creativecommons.org/wiki/Best_practices_for_attribution
For more instruction on how to mark your own content with a CC license, see: https://wiki.creativecommons.org/wiki/Marking_your_work_with_a_CC_license