Section 107 
of U.S. copyright law contains a list of the various purposes for which the reproduction of a particular work may be considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research.
Wikipedia explains: fair use is a limitation and exception to the exclusive right granted by copyright law to the author of a creative work. In United States copyright law, fair use is a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders.
Electronic Frontier Foundation adds: Courts have previously found that a use was fair where the use of the copyrighted work was socially beneficial. In particular, U.S. courts have recognized the following fair uses: criticism, comment, news reporting, teaching, scholarship, research and parodies.
Lawyers disagree about the conceptual nature of fair use. Some lawyers claim that fair use is merely a defense to a claim of copyright infringement. Although fair use is often raised as a defense, many lawyers argue that fair use can also be viewed as having a broader scope than this. If fair use is viewed as a limitation on the exclusive rights of copyright holders, fair use can be seen as a scope of positive freedom available to users of copyrighted material. On this view, fair use is the space which the U.S. copyright system recognizes between the rights granted to copyright holders and the rights reserved to the public, where uses of works may or may not be subject to copyright protection. Copyright law gives the decision about whether copyright law applies to a particular use in this space to a Federal Court judge, to decide after weighing up all relevant factors and the underlying policies of copyright law.