Fair Use in U.S. copyright law is the doctrine that exempts some copyrighted material for the purpose of study, research, and criticism.
In the field of Education, this is a very important concept since sometimes copyrighted material can be used for educational purposes.
Again though, there are legal factors to take into consideration including:
1) The purpose of use (educational - not to be confused with entertainment)
2) What is the nature of the copyrighted material
(See also http://cms.bsu.edu/academics/libraries/collectionsanddept/copyright/fairuseteach/fourfactorsfairuse)
3) How much of the work are you using?
4) What implications will your use have on the owner(s) of the copyrighted work? Put yourself in their shoes...