Share-Alike is a descriptive term used in the Creative Commons project for copyright licenses which include certain copyleft provisions. The Share-Alike license comes in two varieties, CC-BY-SA and CC-BY-NC-SA.
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As all Creative Commons licenses, share-alike licenses require attribution. Share-alike means that all derivatives of a work to be licensed under the same (or a compatible) license as the original. Thus, if a person were to use parts of a BY-SA movie to create a new short film, that new short film would also need to be licensed as BY-SA. According to Creative Commons, the advantage of this license is that future users are not able to add new restrictions to a derivative of your work; their derivatives must be licensed the same way.[1] The Share-Alike license is similar to the GNU Project's General Public License (GPL) and Free Documentation License (GFDL).
There are two varieties of Share-Alike licenses. The first, CC-BY-SA, allows commercial use. The second, CC-BY-NC-SA, which requires non-commercial use, only is Share-Alike but is not free content (and not copyleft, according to some critics).[2]
Creative Commons Attribution-ShareAlike licenses are based on the principles of the Free Software movement. As applied to content, these principles require a license to grant the following essential freedoms to ALL users of licensed works:
These 4 freedoms are taken directly from the Definition of Free Culture Works
By contrast, there are also many permissive free software licences which do not require share-alike terms to be applied, thus permitting users to make modifications and improvements and apply a modified and more restrictive license.
Over the years, Creative Commons has issued 4 versions of the BY-SA and BY-NC-SA licenses (1.0, 2.0, 2.5, and 3.0).
In June, 2009 the Wikipedia community and Wikimedia Foundation board approved the adoption of the Creative Commons Attribution-ShareAlike (CC BY-SA) license as the main content license for Wikipedia and other Wikimedia sites.[3] This decision was hailed by many as a win for Free Culture as well as visionary leadership.[4]