From Sandbox

Using copyrighted work from others

All works are copyrighted unless they either fall into the public domain or their copyright is explicitly disclaimed.

If you use part of a copyrighted work under "fair use", or if you obtain special permission to use a copyrighted work from the copyright holder under the terms of our license, you must make a note of that fact (along with names and dates).

Never use materials that infringe the copyrights of others. This could create legal liabilities and seriously hurt the project. If in doubt, write it yourself.

Note that copyright law governs the creative expression of ideas, not the ideas or information themselves. Therefore, it is legal to read an encyclopedia article or other work, reformulate the concepts in your own words, and submit it. However, it would still be unethical (but not illegal) to do so without citing the original as a reference. See plagiarism and fair use for discussions of how much reformulation is necessary in a general context.

Linking to copyrighted works

External sites can possibly violate copyright. Linking to copyrighted works is usually not a problem, as long as you have made a reasonable effort to determine that the page in question is not violating someone else's copyright. If it is, please do not link to the page. Knowingly and intentionally directing others to a site that violates copyright has been considered a form of contributory infringement in the United States (Intellectual Reserve v. Utah Lighthouse Ministry). Also, linking to a page that illegally distributes someone else's work sheds a bad light on us. If the site in question is making fair use of the material, linking is fine.