The U.S. law that provides for [*fair use*](http://copyright.gov/fair-use/) is [17 USC § 107](https://www.law.cornell.edu/uscode/text/17/107).

Canonical answers here:

* http://law.stackexchange.com/questions/7683/in-the-us-when-is-fair-use-a-defense-to-copyright-infringement
* [Case law and test for whether something qualifies as fair use](http://law.stackexchange.com/questions/2108/is-it-fair-use-to-discuss-a-fictional-work-on-a-webpage/3431#3431)
* [History of fair-use law](http://law.stackexchange.com/a/11413/10)