Copyright laws. It all depends on the question. Rather than try to cover everything there is about copyrights - which has already been done, I am going to list some of the resources I found in researching copyright laws.
Get copyright permissions HERE
Using trademarks in fiction - a publishing and intellectual property attorney's point of view
More trademarks in fiction - from a publishing company point of view
A chart of copyright terms and public domain in the US
An easy to use site with tons of links and resources.
Remember, citing a source is not "permission" to use someone else's work. Good luck and write on!