"Creative Commons is a nonprofit organization that helps overcome legal obstacles to the sharing of knowledge and creativity to address the world’s pressing challenges." - What We Do page on the Creative Commons website
The greatest contributions of Creative Commons have been the development of standardized, internationally recognized, digital intellectual property licenses and the public domain resources they have made available for free. A "Digital Commons" as they have titled it, that is uniquely identified by the 
symbol.
As creators and users of academic and educational materials, we believe in the importance of sharing knowledge and making it as accessible as possible. To encourage this, Creative Commons established the 5Rs of Open Educational Resources: Retain, Revise, Remix, Reuse, and Redistribute. Learn more about Open Educational Resources in our new OER LibGuide. These are essential to students and educators who are looking for free educational materials that they can download, share, and modify.
Creative Commons licenses are not a replacement for copyright, nor are they against or contrary to traditional copyright laws. They are an additional layer of licensing on top of copyright that extends into the digital realm, where they make the parameters of reuse more accessible to users. Basically, it allows the author of a creative work to more clearly define how it can be utilized by others.
"CC licenses are copyright licenses, and depend on the existence of copyright to work. CC licenses are legal tools that creators and other rights holders can use to offer certain usage rights to the public, while reserving other rights. Those who want to make their work available to the public for limited kinds of uses while preserving their copyright may want to consider using CC licenses." - FAQ page on the Creative Commons website
There are six different license types as well as a public domain option. The Creative Commons website also provides a free resource for ensuring content creators select the right one for their needs. All licenses are free to use and are legally binding.
"How to License Poster" by Creative Commons is licensed under CC BY 4.0
The Creative Commons License Options
Before Selecting a License for your Work, Consider These Things
Choosing a License for Your Work
*REMINDER* The term Creative Commons does not mean free to use with no strings attached.
Once you discover that a work has a Creative Commons license, that does not say Public Domain, it is important that you give credit to the original creator by attributing your source. These "citations" look very different than the traditional academic formats such as MLA, APA, Chicago, etc... Creative Commons has created a best practices wiki to show users how to properly attribute sources digitally. 
The non-profit group has even authored a book/ebook called Made With Creative Commons that can be downloaded for free on their website in multiple formats or ordered on Amazon.