Copyright is the right of an author or creator of a work to control how people use his or her work. In the United States, copyright law gives authors and creators the exclusive rights to copy and/or distibute their work, to create derivative works, and to publicly perform or display their work.
United States copyright law also gives several exceptions to the authors and creators rights so that copyright doesn't prevent the use of works for things like teaching and scholarship. These exceptions include first sale, which allows you to lend or sell your copy of a copyrighted work, exceptions for classroom use, which allows students and teachers to display images and play videos as part of a class, and fair use. Fair use is designed to allow use of works for purposes like teaching, scholarship, research, and news reporting. Fair use requires the evaluation of four factors about the proposed use to determine if the use is fair.
For a more information on Copyright, visit the website of the United States Copyright Office.
These websites provide both overviews of copyright and indepth explanations of the complex issues involved.
A list of tools designed to help you know when and how you can use copyrighted works