Stanford University offers a wonderful set of resources related to copyright law and protection. Here is their section on fair use where you should find just about everything you need to know.
Source: Stanford University
Explore this page from the Library of Congress. It provides a brief overview of copyright law and then explains fair use and includes a number of examples especially related to educational uses of protected content.
Source: Library of Congress
One common practice today is the use of media including video, images, music and sound to create new works. This practice is called remixing and is widespread in our culture today. You cannot go to Youtube and not run into remixed media. From time to time these works are removed for copyright violation. There is a fine line between fair and unfair use. The Center for Social Media has created a video that illustrates the widespread remixing of media and discusses the limits of fair use.
Below is a comic book created by Duke University's Center for the Study of Public Domain. It is a great and creative approach to communicating the laws and ideas behind copyright, fair use and public domain.