Works in the public domain lack copyright protections and, therefore, can be used in anyway you like, although the scholarly honor code still requires your reference the author through citation.
Works enter the public domain in various ways:
These tools will help you understand the copyright status of any work created in the United States:
Copyright Genie: An interactive tool that tells you the copyright status of a work based on age and policy.
Copyright Term and the Public Domain in the United States A chart that layout all the details of U.S. copyright law with respect to the protections for any given work.
Digitization has increasingly made works in the public domain broadly and freely available. Below are some select digital collections that include textual, visual, and audio sources. For a more complete list, visit Wikipedia: Public Domain Resources.
Digital Public Library of America
Library of Congress Digital Collections
Creative Commons licenses represent a departure from traditional copyright, which automatically reserves all rights for the creator of a work. By contrast, Creative Commons licenses offer users the ability to retain, revise, reuse, remix, and redistribute their work, depending on which license is chosen.
The infographic below explains the six individual licenses. For a comprehensive explanation of each license, use this Choose a License tool.
(Image credit: Ohio State University Libraries, Copyright Corner)