Creative commons is a non-profit organisation that works toward making content available for free use, sharing and remixing.
It is responsible for developing the licensing terms for a lot of the content that is available outside conventional copyright. The CC licenses explain how creators want to share their work, and what rights they want to give the users.
There are licenses for 'some rights reserved' to 'no rights reserved' or 'all rights reserved.' Some attributes of the licenses which individuals can choose (in combination) are:
| BY - Attribution i.e the work must be attributed | NC - Non Commercial i.e the work may be reused but only for non - commercial purposes. | ND - No Derivatives i.e the work may be reused or distributed in the original form only. | SA - Share Alike i.e any derived work must be licenced under same or similar terms. | 
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CC also allows for work to be released with the author waiving all rights. It also can help identify works that are in the public domain. For details on what you can and cannot do, do visit the Creative Commons website.
Other alternatives to copyright licenses include copyleft, GNU General Public license, and even (go figure) WTFPL.