Fair use is a doctrine of US Copyright Law, allowing for the limited use of copyrighted material without requiring permission from the copyright holder. It permits legal, non-licensed citation or incorporation of copyrighted material in another creator's work using a 4-part balancing test. The tricky part about Fair Use is that the balancing test is subjective and open to interpretation.
The four factors judges consider are:
Source: University of Wisconsin-Madison Libraries.
Prof. Eric Faden of Bucknell University (fairly) compiled clips from Disney movies to create a video explaining Fair Use. Double-click the video to see it larger in You Tube. (source: Univ. of Wisconsin - Madison libraries)
Do you ever wonder why:
Copyright is a federal law that gives creators of media the exclusive rights to copy, distribute, and mash up the things they create for a limited time. As you work on any assignment that includes digital media (e.g., images, video, or music) it is good practice to make sure you are abiding by copyright laws.
Source: University of Wisconsin-Madison Libraries
Check out some more links of interest from a more detailed copyright guide.