Creative Commons is a non-profit organization founded in 2001 that, over the years, has released a set of licenses that enable creative types to share their work with others. The content creator allows others to use their work, just as long as the users follow the guidelines set forth in that particular license. It’s a “some rights reserved” system rather than an “all rights reserved system.”
There are four parts to a CC license, three of which you can choose to include or leave out. The initial stipulation that is always there is Attribution. This basically states the user must credit you when using your work. The three remaining stipulations that you can add to Attribution are: No Derivatives, Non-Commercial and Share Alike.