What is Creative Commons?
Creative Commons is a nonprofit organization that has created licenses that work in conjunction with copyright law to allow creators to specify how they will allow others to use their works without seeking permission. You can use works licensed under Creative Commons without seeking permission from the copyright holder as long as you follow the license terms. You may also choose to license your own works with a Creative Commons license in order to allow others to use them.
Creative Commons licenses are legally recognized licenses. They do not supersede copyright. All copyright laws including the ability to use works under fair use still apply. However, the licenses allow copyright holders to extend beyond copyright limitations to allow others to do such things as copy, reuse, remix, edit, share, and build upon their works.
For more information on Creative Commons visit their website.
Creative Commons License Types
Attribution (CC BY)
The Attribution only license allows others to do whatever they want with a work including using it for commercial puproses as long as they attribute it back to the original creator. It is the least restrictive of the Creative Commons licenses.
Attribution ShareAlike (CC BY-SA)
The Attribution ShareAlike license allows others to do whatever they want with a work including using it for commercial purposes as long as they attribute the original work back to the creator and license any new works based upon the original work under the same Creative Commons license.
Attribution-No Derivatives (CC BY-ND)
The Attribution-No Derivatives license allows others to copy and share a work even for commercial purposes. However, they must not change the work in any way, and they must attribute it back to the original creator.
Attribution-NonCommerical (CC BY-NC)
The Attribution-NonCommerical license others others to do whatever they want with a work as long as it is not for commercial purposes and they attribute the work back to the original creator.
Attribution-NonCommercial-ShareAlike (CC BY-NC-SA)
The Attribution-NonCommercial-ShareAlike license allows others to do whatever they want with a work as long as it is not for commercial purposes. They must also attribute the work back to the original creator, and if they create any new works based on the original work they must license their works under the same Creative Commons license.
Attribution-NonCommercial-NoDerivatives (CC BY-NC-ND)
The Attribution-NonCommercial-NoDerivatives license is the most restrictive of the Creative Commons licenses. It allows others to copy or share the work without making any changes for non-commerical purposes as long as they attribute it back to the original creator.
CC0
The CC0 license attempts to allow creators to completely relinquish their copyrights. Many copyright laws including the United States' automatically grant copyright whether or not a creator requests it. The CC0 license lets a creator indicate that that they are not reserving any of their rights under copyright law, thus anyone can use the work in any way that they wish. Some legal jurisdictions may not allow for works to be entered into the public domain before their copyright term has expired. The CC0 license indicates that creators will treat their works as if they are in the public domain even when the law does not allow them to be.
Attribution
For information on attribution and examples, see Creative Common's wiki page "Best Practices for Attribution."