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 the Creative Commons Website.
There are several kinds of CC License, some of which are more restrictive than others. Because of this, it is important to review the terms of the license for any media that you want to use. Keep in mind that if you plan to alter the work in any way (include an image in a video, for instance, or modify it using Photoshop) the license must be one that allows you to make derivatives. You can find more information about the various CC Licenses here.
Be aware of the Share-Alike license, which requires you to use a similar license in your end-product that contains the Share-Alike work.