Fair use is a doctrine in United States copyright law that allows limited use of copyrighted material without requiring permission from the rights holders, such as use for scholarship or review. It provides for the legal, non-licensed citation or incorporation of copyrighted material in another author's work under a four-factor balancing test. The term "fair use" originated in the United States, but has been added to Israeli law as well; a similar principle, fair dealing, exists in some other common law jurisdictions. Civil law jurisdictions have other limitations and exceptions to copyright.
The first factor is about whether the use in question helps fulfill the intention of copyright law to stimulate creativity for the enrichment of the general public, or whether it aims to only "supersede the objects" of the original for reasons of personal profit. To justify the use as fair, one must demonstrate how it either advances knowledge or the progress of the arts through the addition of something new. A key consideration is the extent to which the use is interpreted as transformative, as opposed to merely derivative.
The third factor assesses the quantity or percentage of the original copyrighted work that has been imported into the new work. In general, the less that is used in relation to the whole, e.g., a few sentences of a text for a book review, the more likely that the sample will be considered fair use.
The fourth factor measures the effect that the allegedly infringing use has had on the copyright owner's ability to exploit his original work. The court not only investigates whether the defendant's specific use of the work has significantly harmed the copyright owner's market, but also whether such uses in general, if widespread, would harm the potential market of the original. The burden of proof here rests on the defendant for commercial uses, but on the copyright owner for noncommercial uses.
http://en.wikipedia.org/wiki/Fair_use
Go ahead, flunk me.