Note that the author of this guide is not an attorney and is providing information on copyright only. Nothing on this site should be taken as legal advice. For legal advice, please see an attorney.
The public domain consists of works that are not copyrighted. These works are either old enough that copyright has expired on them or they were not copyrightable in the first place. These works can be used much more freely the copyright protected works. Even works in the public domain need to be cited - in other words, they are not free of ethical issues, only legal issues.
Anything published before 1923 is in the public domain. You can republish a book or rewrite it and create a derivitive work from it.
These works are also in the public domain and can be used. Because unpublished works have a different copyright term than published works, the date before which all unpublished works are in the public domain is much earlier than for published works.
All works created by U.S. government employees in the course of their jobs are copyright free from the time they are created. Some states also put the works of their employees into the public domain (although South Dakota does not). Not all government documents are in the public domain because some of them are created by entities other than government employees. For instance, a report might be written by an SDSU faculty member working on a grant from a federal agency. The document was not written by a U.S. government employee in the course of their job so it is not automatically in the public domain even if the U.S. government publishes it. Also, a person might work for the U.S. government during the day and go home and write the next great American novel at night. Because the novel was not written in the course of the person's job, it is also not in the public domain.
Some people feel that some or all of their work should be usable by others without limitation and they place their work in the public domain. The most common way of doing this is to use a Creative Commons license. Some copyright experts question whether this is legally placing a work in the public domain but one must assume that if a creator does this, they are unlikely to sue for copyright violation later. There is more information on the Creative Commons under the Licensing tab.