Creative Commons (CC) licenses are a way (though not the only way) for owners of copyrighted material to, in essence, approve certain types of uses of their work without permission. Creative Commons licensed materials (except those explicitly licensed with a "Public Domain" license) are still copyrighted and the creator of the work still owns that copyright.
The purpose of this licensing is to offer a flexible and efficient mechanism for content creators to allow their materials to be used in certain ways without the burden of the permissions process.
Typically, CC licenses require attribution at a minimum. Copyright holders can choose licenses that allow (or prohibit) commercial uses, declare whether adapting the work is permitted, and to require the user to also use a CC-license on derivative works.
Creative Commons for Educators and Librarians [PDF] by