Fair Use allows limited use of copyrighted materials for educational and research purposes.  The statute outlines four factors that should be assessed in determining whether a use is a Fair Use.  These are:

• the purpose and character of your use
• the nature of the copyrighted work
• the amount and substantiality of the portion taken, and
• the effect of the use upon the potential market.

The test is, intentionally, fact dependent, so you need to look at each use individually to determine if it is a fair use. 

Stanford’s Copyright & Fair Use site (http://fairuse.stanford.edu) provides much more detail on fair use, its application, and relevant case law.