Creative Commons is a nonprofit organization that created a set of simple, easy-to-understand copyright licenses. Rather than limiting works to "All Rights Reserved" as entailed in traditional copyright, CC Licenses seek to clarify and expand to "Some Rights Reserved." Their mission is to "increase the amount of creativity (cultural, educational, and scientific content) in 'the commons' -- the body of work that is available to the public for free and legal sharing, use, repurposing, and remixing."
There are six Creative Commons Licenses which are machine-readable (for computers), human-readable (for humans), and lawyer-readable (for the legally minded). Each license requires attribution through CC-BY. From there they become more and more "closed" with CC-BY-NC-ND which allows for the work to be freely distributed but it cannot be adapted or commercially marketed/sold.
Licensing your work
With Creative Commons licensing terms, if you own the copyright to your work, you can decide how others can that work by assigning a free Creative Commons license to it. This step is completely optional. Your choice of licenses determines:
Attribution (giving credit in a way you establish)
Sharing (those who use your content must use the same type of license you do)
Type of use (commercial vs non-commercial)
Derivatives (determines whether someone can make a derivative work from yours)