Creative Commons is a nonprofit organization that makes it easier to share, reuse, repurpose and remix creative material by providing free licenses to let creators give permission in advance for certain uses of their material. 

Creative Commons licenses are legal tools that let people who own copyrights pre-approve certain uses of their works. They can be helpful for creators who want their works widely used, and they can be helpful for people who want to find things that are free to use with clear permissions. The licenses are not an alternative to copyright--they are an option for sharing works, and they rely on copyright.

There are six licenses offered by Creative Commons, listed from most open (CC BY) to least open (CC BY-NC-ND), indicating how much freedom others have in using your work.

"Ordering of Creative Commons licenses from most to least permissive," by Stephan Kulla, creativecommons.org, under the Creative Commons Attribution-Share Alike 4.0 International.

Attribution (CC BY): lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation. 

Attribution-ShareAlike (CC BY-SA): lets others remix, tweak, and build upon your work even for commercial purposes, as long as they credit you and license their new creations under the identical terms. All new works based on yours will carry the same license, so any derivatives will also allow commercial use. 

Attribution-NoDerivs (CC BY-ND): allows for redistribution of the complete, unchanged work for commercial and non-commercial purposes, with credit to you.

Attribution-NonCommercial (CC BY-NC): lets others remix, tweak, and build upon your work non-commercially

Attribution-NonCommerical-ShareAlike (CC BY-NC-SA): lets others remix, tweak, and build upon your work non-commercially, as long as they credit you and license their new creations under identical terms.

Attribution-NonCommercial-NoDerivs (CC BY-NC-ND): only allows others to download your works and share them with others as long as they credit you, but they can’t change them in any way or use them commercially.

• Visibility and spread: Works that are widely available get wide use! Creative Commons licenses allow your work to spread further by making it clear that re-use is encouraged. Also, the Creative Commons licenses can be machine readable. If you mark your licensed works with HTML code provided by Creative Commons, search engines know how it can be reused.
• Easier access: Releasing your work under a Creative Commons license decreases financial barriers to access. It can also help decrease technical barriers. Organizations working to increase access will be able to aggregate and reformat your work.
• Saving time: Processing permissions requests can take time for a copyright holder and takes time on the side of the person requesting permissions. Once you've applied one of the Creative Commons licenses to your work, you can refer all potential users to the License Deed.