Have you wondered why certain books are available to view in their entirety online through such sites as Google Books and others are not? Have you wanted to photocopy an entire book or manuscript at a library and been told that you can't because of copyright laws? Have you made online postings that include information from someone else's publication?
Most of us know that anything published before 1923 is in the public domain. In addition, anything published in the U.S. from 1923-1977 without an official copyright notice is now in the public domain. Likewise, anything published from 1923-1963 with copyright notice is in the public domain if copyright was not renewed. To learn how to find out whether an item's copyright has been renewed, visit the FAQ on The Online Books Page.
Did you know that unpublished works (such as funeral home records) actually have longer protection than copyrighted material, not coming into the public domain for 120 years? What does this mean to the researcher? Publishing copies or abstracts of these items, online or off, is prohibited without the express permission of the record holder.
The good news about things published by the U.S. Government is that they immediately go into the public domain, so watch for the publisher name of GPO (Government Printing Office).
Cornell University's Copyright Term and the Public Domain page does a nice job of breaking down copyright law, discussing published, unpublished, and government materials. Make a point of familiarizing yourself with the laws to protect yourself and those whose work from whom you draw. And, you may discover that more things are in the public domain than you thought.