When a work is no longer protected by intellectual property rights (e.g., copyright, trademark) it is considered a part of the public domain. No single person owns the public domain; as the name implies, it is available to everyone. This means that you do not need permission to use, adapt, or distribute these works.
There are four ways that works commonly rise into the public domain:
Look at all of these methods in depth at Welcome to the Public Domain from the Stanford University Libraries.