Fair use is a concept embedded in U.S. law that recognizes that certain uses of copyright-protected works do not require permission from the copyright holder or its agent. These include instances of minimal use that do not interfere with the copyright holder's exclusive rights to reproduce and reuse the work.
Fair use is not an exception to copyright compliance; it is more of a "legal defense." That is, if you copy and share a copyright-protected work and the copyright holder claims copyright infringement, you may be able to assert a defense of fair use which you would then have to prove.
Fair use is primarily intended to allow the use of copyright-protected works for commentary, parody, news reporting, research and education. However, not all uses in an academic context are automatically considered fair use.
The Copyright Act does not spell out the specific types of content reproduction that qualify as fair use. It offers an outline as to how to analyze whether fair use may apply in a particular situation. As a result, the Copyright Act leaves it up to the individual to determine, based upon the factors in Section 107 of the Act, whether fair use applies in each particular circumstance. To avoid a potential legal challenge from the copyright holder, many institutions follow a policy of "when in doubt, obtain permission."