The United States Copyright Office publishes a circular titled "Copyright Basics." This is a good place to begin with questions that you have about copyright law. The Copyright Office also makes other copyright information circulars and form letters available on the Web.
Copyright Basics is also the name of an animated six minute video. Created in 2009 by the Copyright Clearance Center, this video details how to share copyrighted material at work while still respecting the rights of content creators.
Map of Use Issues from the University of Minnesota poses a series of yes/no questions to aid in determining copyright questions for a particular work.