Public domain includes all works that are past their period of copyright protection, that have never been covered by copyright, or that have been donated to the public domain by their creators. These works can be used free of charge and without permission of their creators.
A good rule of thumb is that any work that was published in the US before 1923 is in the public domain. Unpublished works generally enter the public domain 70 years after the death of the author.
Other materials covered by public domain include facts, ideas, numbers, and any work created by the US Government.
Note: Just because a work is out of print or available free of charge does not mean it is in the public domain.
There are plenty of exceptions and complications to public domain – a good resource to determine the public domain status of a particular work is the Digital Copyright Slider.