Public Domain materials are works that are free from intellectual property laws. There are no restrictions for individuals to use these works, nor are permissions necessary. Works within the Public Domain can never be owned by any individuals. There are different ways that works become part of the Public Domain. These include:
Expiration of copyright period (please see Cornell University's Copyright and Public Domain Guide)
Failure to comply with copyright formalities
Intentional dedication to Public Domain by the creator
Federal employees' works
"Common heritage of sources, or folk customs."
You can use any U.S. work in the Public Domain in any way that you want, as much as you want. They belong to the public. However, citation is still required for remixing and reusing public domain works. Additionally, keep in mind that copyright laws and designations of the public domain vary by country.
Excellent tool for determining the duration of copyright for a document.
Detailed chart developed by Peter Hirtle at Cornell for determining whether or not a document is in the public domain.