When a copyright term expires, or is given up by the owner, the work falls into the public domain (U.S. Copyright Office 1992). The public can now use the work (music, literature, art, photos, etc.) freely to create, perform, or make derivatives without permission.
The following are examples of works that are in the Public Domain:
Works published in the United States prior to 1923
Works published in the United States between 1923 - 1977 without a copyright notice
Unpublished work by an author who died 70 years prior (Example, author dies in 1944, unpublished works falls into public domain in 2015.)
Unpublished anonymous and pseudonymous works, and works made for hire (corporate authorship) that are more than 120 years old
Works donated to the Public Domain
Most Government Publications
Many of these works can be found on the Project Gutenberg website which offers over 46,000 free ebooks that can be downloaded or read them online.
If you're unsure if a particular work is in the public domain, check the Public Domain Slider, a tool that will help you determine if a work you're considering using is in the Public Domain.