Fair use provides for the public to use portions of copyrighted material "for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" without infringing on the copyright holder's rights.
17 U.S.C. § 107
It was developed in 1976 to allow use of material in an educational environment. This is an umbrella term that should not be considered all encompassing as other laws and restrictions may over-rule.
The four factors that judges use to determine if a use falls under fair use are:
17 U.S.C. § 107
For another perspective of Copyright and Fair Use using Disney movies as an example, please view this 10 minute video.