Creative Commons is a licensing system designed to make it easier for people to share their open access works with others. A Creative Commons license is a special form of copyright license that is used by creators to indicate that the public may use, share, and sometimes even build upon their work. For more information, see Creative Commons Website.
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There are several kinds of CC Licenses, some of which are more restrictive than others. It is important to review the terms of the license for anything that you want to use. Keep in mind that if you plan to alter a CC licensed work in any way (for instance, modify a photo in Photoshop, edit a video, etc.) the license must be one that allows you to make derivatives. You can find more information about the various CC Licenses here.
Always check rights carefully! Note that some images with Creative Commons licenses are derived from sources that are protected by copyright.