"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."
"This “fair use” provision of copyright law doesn’t provide hard and fast rules to tell you whether a use qualifies as fair. Instead, the unique facts regarding a use lead you to a reasoned conclusion. Your evaluation should weigh four factors:
1. Purpose and character: If your use is for teaching at a nonprofit educational institution, this is a factor favoring fair use. The scale tips further in favor of fair use if access is restricted to your students.
2. Nature of copyrighted work: Is the work fact-based, published, or out-of-print? These factors weigh in favor of fair use.
3. Amount used: Using a small portion of a whole work would weigh toward fairness. But sometimes it may be fair to use an entire work (such as an image) if it is needed for your instructional purpose.
4. Market effect: A use is more likely to be fair if it does not harm the potential market for or value of the copyrighted work. But if it does, this could weigh more heavily against fair use than the other factors.
Source: Know Your Copy Rights, Association of Research Libraries