Wikipedia Reference Information
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. It is based on free speech rights provided by the First Amendment to the United States Constitution. The term "fair use" is unique to the United States; a similar principle, fair dealing, exists in some other common law jurisdictions. Civil law jurisdictions have other limitations and exceptions to copyright.
United States trademark law also incorporates a "fair use" defense. While the names are the same, the doctrines are quite different.
The complete, up-to-date and editable article about Fair Use can be found at Wikipedia: Fair Use