The public domain consists of works that are ineligible for copyright protection, works with expired copyrights, or those that have been placed in the public domain by the creator. These materials are owned by no one and are considered "public property." As such, they are available for anyone to use without permission. But remember - just because something is in the public domain doesn't mean you shouldn't cite it. Always give proper credit to your source!
In the United States, the following are generally in the public domain:
Use the public domain "slider" from the ALA Copyright Toolkit. (Slider requires Flash Player to be downloaded).
Duration of copyright is related to public domain. For works created after 1978, copyright protection lasts for the life of the author plus seventy years. For works created before 1978, different duration amounts, terms, and requirements exist.
Copyright is meant to protect the rights of creators and to encourage the creation of science and scholarship. In most cases, you must ask permission of the copyright holder to use his or her work, but there are limitations and exceptions to copyright law: Most importantly for educators, you do not need to request permission of the copyright holder if your use of a work is considered a "fair use."
Section 107 of the U.S. Copyright Law says that the fair use of a copyrighted work "for purpose such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" does not infringe on copyright.
To determine whether your use of an item is fair, you'll need to consider the four factors of fair use and do a fair use analysis:
Factor 1: Purpose and character of the use
Factor 2: Nature of the work
Factor 3: Amount of the copyrighted work used
Factor 4: Effect of the use upon the market