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).

The UK has [fair dealing](https://en.wikipedia.org/wiki/Fair_dealing_in_United_Kingdom_law) instead of fair use. ([UK gov. description](https://www.gov.uk/guidance/exceptions-to-copyright)). It is also an [affirmative defence](https://en.wikipedia.org/wiki/Affirmative_defense). It is less permissive than the fair use standard in the US, requiring that the re-use be from one of four categories of use, and then that that use *also* be fair (this second step roughly follows the US test).

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)
* Regarding "[Transformative Use](https://law.stackexchange.com/a/31678/10)"