Public Domain refers to works whose exclusive intellectual property rights have expired, been forfeited or have been waived or are inapplicable. Rights vary by country.
Every book or story written prior to 1923 is in the public domain.
A film that was released to public domain by its author or its copyright has expired. Due to the US Copyright Term Extension Act, no more films will automatically enter the public domain in the US until January 1, 2019, when the copyright will expire on films released in 1923. Then in 2020, films from 1924 will enter the public domain.
In the United States, any musical works published before 1922, in addition to those voluntarily placed in public domain, exist in the public domain. In most other countries, music generally enters the public domain in a period of fifty to seventy-five years after the composer's death.