From Sonic Retro
The license Sonic Retro uses grants free access to our content in the same sense as free software is licensed freely. This principle is known as copyleft. In other words, this content can be copied, modified, and redistributed so long as the new version grants the same freedoms to others and acknowledges the authors of the article used (a direct link back to the article satisfies our author credit requirement). Articles therefore will remain free forever and can be used by anybody subject to certain restrictions, most of which serve to ensure that freedom.
To fulfill the above goals, the text contained in Sonic Retro is licensed to the public under the GNU Free Documentation License (GFDL).