The below information is directly from the U.S copyright office.  This site is for informational and learning purposes only and does not profit in any way. A Link to the original work is always posted with the excerpted material.

“Fair use is a legal doctrine that promotes freedom of expression by permitting the unlicensed use of copyright-protected works in certain circumstances. Section 107 of the Copyright Act provides the statutory framework for determining whether something is a fair use and identifies certain types of uses—such as criticism, comment, news reporting, teaching, scholarship, and research—as examples of activities that may qualify as fair use.  Section 107 calls for consideration of the following four factors in evaluating a question of fair use:

• Purpose and character of the use, including whether the use is of a commercial nature or is for nonprofit educational purposes:  Courts look at how the party claiming fair use is using the copyrighted work, and are more likely to find that nonprofit educational and noncommercial uses are fair.  This does not mean, however, that all nonprofit education and noncommercial uses are fair and all commercial uses are not fair; instead, courts will balance the purpose and character of the use against the other factors below.  Additionally, “transformative” uses are more likely to be considered fair.  Transformative uses are those that add something new, with a further purpose or different character, and do not substitute for the original use of the work.
• Nature of the copyrighted work:  This factor analyzes the degree to which the work that was used relates to copyright’s purpose of encouraging creative expression. Thus, using a more creative or imaginative work (such as a novel, movie, or song) is less likely to support a claim of a fair use than using a factual work (such as a technical article or news item). In addition, use of an unpublished work is less likely to be considered fair.”