Fair Use is an exemption to copyright law that allows the use of copyrighted works without permission or payment of fees in order to encourage teaching, learning, and scholarship - such as for a class presentation or paper.
A Fair Use determination must be made for each intended use, and the answer may still be vague or murky. The only way to get a definitive answer on Fair Use is for a case to go to court and receive a ruling from a judge. However, this very rarely happens.
The four factors, or principles, of Fair Use help determine whether or not a potential use is a Fair Use. These factors are:
Purpose or character of the use
Nature of the work being used
Amount of the work being used, and its substantiality in relation to the whole
Effect of the intended use on the market