What is public domain?
Public domain refers to works that are not protected by intellectual property laws such as copyright. These works can be freely used by the public without obtaining any specific permissions.
How do works become public domain?
Copyright isn't forever; current laws states that for works published after 1977, the copyright will expire 70 years after an author's death. All works published before January 1923 are currently public domain; due to a change in the law in 1998, works published in 1923 will not become public domain until 2019. After that, works published in 1924 will become public domain in 2020, works published in 1925 will become public domain in 2021, and so on.
Work can also become part of the public domain if the copyright holder has not been renewed (this applies mostly to works published from 1923 to 1963); if the copyright holder places the work in the public domain; or the work is not subject to copyright protection (for example, government documents produced for public distribution).
How do I know if a work is in the public domain?
Unless the copyright status is explicitly stated or you're certain a work was produced before January 1923, this generally requires some research or detective work. At the very least, you'll need to know the date of creation and the country of origin, then determine the applicable copyright law. This extremely helpful table produced by Cornell University can help you determine the copyright status of various works.
If the creator/copyright holder is unknown, is a work automatically part of the public domain?
No, and orphan works constitute a huge number of copyrighted material in the U.S. Legislation dealing with the management of orphaned materials has been proposed, but never passed, in the U.S., and several notable lawsuits concerning the appropriation of orphaned works have been filed in recent years. However, no clear resolution has been established to date, and many individuals and institutions will not display, broadcast, digitize, or otherwise use orphaned works for fear of being sued by the copyright holder.
Cornell University has produced a handy table explaining when various works became or will become part of the public domain. Copyright expiration varies based on type of work, date of publication, and country of origin, among other factor. Copyright Term and the Public Domain