When copyright ownership/protection expires, the work enters the public domain meaning anyone can freely use the work without seeking permission or paying a fee.
Both U.S. and international copyright laws governing the length of copyright protection have been revised several times, thus it is often difficult to accurately determine if a work has entered the public domain. See the following chart Copyright Term and the Public Domain in the United States for determining if a work has entered the public domain. Also, here is a handy tool to answer the question, Is It Protected by Copyright?.
The Creative Commons is another great resource for content that is available without cost to use and adapt with attribution. The Creative Commons is a licensing mechanism that enables content creators to easily share content for reuse. Creative Commons licenses and website search tools can provide clear guidance about acceptable and legal uses of digital content to create and share materials. These digital resources can include images, music audio files, movies, or any other type of media. Learn More