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Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools. It is basically a means of establishing 'some rights reserved' instead of the default copyright 'all rights reserved.' It is NOT an alternative to copyright, but instead something that is applied on top of copyright, modifying copyright terms to best suit the content creator's needs.
BY - Attribution: All Creative Commons license require that use of the work in any way (copy, distribute, display, perform, make derivatives/remixes, …) must give the creator credit the way they request, but not in a way that suggests that the creator endorses them or their use. If they want to use the work without giving the creator credit or for endorsement purposes, they must get the creator’s permission first.
SA - ShareAlike: This selection allow others to copy, distribute, display, perform, and modify thework, as long as they distribute any modified work on the same terms (“not more restrictive”). If they want to distribute modified works under other terms, they must get the creator’s permission first. Without share-alike, derivative works might be sublicensed with a more restrictive license.
ND - NoDerivs: This allows others to copy, distribute, display, and perform only original verbatim copies of the work. If they want to modify your work, they must get the creator’s permission first. No derivative works or remixes are permissible with this option.
NC - NonCommercial: This option allows others to copy, distribute, display, perform, and (unless the creator has chosen NoDerivs) modify and use the work for any purpose other than commercially unless they get permission first.
Various license combinations are possible based on these. There are six general types: