All content is licensed under the Creative Commons Attribution - NonCommercial - ShareAlike license (abbreviation: CC-BY-NC-SA) v2. Again a simple explanation:
Your rights:
You may read the content
You may change the content
You may redistribute the content
Your duties:
If you redistribute you have to do it under the same terms
You may not use the content commercially (eg. printing a book and sell it)
If you redistribute the content you have to say who the original author is including a link to it