Copying, modifying, displaying, performing or distributing another's work may be fair use but far from clear. Hopefully, the following Web sites will give you a better understanding and provide effective guidance for the use of others' works today.
Copyright Term and the Public Domain Works in the United States
"A public domain work is a creative work that is not protected by copyright and which may be freely used by everyone. Works enter the public domain in several ways: When the copyright expires; If a person has placed it in the public domain; If it does not qualify for copyright protection".
“The public domain is the realm of material — ideas, images, sounds, discoveries, facts, texts — that is unprotected by intellectual property rights and free for all to use or build upon. It includes our collective cultural and scientific heritage, and the raw materials for future expression, research, democratic dialogue and education.”
Source: Duke University's Center for the Study of the Public Domain
Fair Use: Checklists & Other Decision Tools