Fair use is a exception in the copyright law that allows users to use a copyrighted work, without permission, under certain circumstances. Fair use decisions are made based by evaluating the use in light of the four factors, which are listed below. 

17 U.S. Code § 107 - Limitations on exclusive rights: Fair use

Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—

1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;

2. the nature of the copyrighted work;

3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and

4. the effect of the use upon the potential market for or value of the copyrighted work.

The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.

Fair Use Checklist (Columbia University Libraries)

This PDF tool helps make fair use assessments by working through each of the four factors. 

Summaries of Fair Use Cases (Stanford University Libraries)

This website provides summaries of fair use cases, which are useful for understanding how fair use decisions are made. 

Codes of Best Practices in Fair Use (Center for Media & Social Impact)

This series of Codes of Best Practices help illuminate the application of fair use in different areas like documentary filmmaking, academic and research libraries, visual arts, online video, and journalism.

Fair Use Index (U.S. Copyright Office)

The index tracks fair use decisions made by U.S. courts.