Public domain works are not restricted by copyright and do not require a license or fee to use. In music, public domain status allows the user unrestricted access to a composition. When the copyright of a song expires, it automatically goes into the public domain. You can find out more about United States copyright law here, and Canadian copyright here. For all other territories, check with you local government.
What works have expired into the public domain?
All works published in the U.S. before 1923.
All works published with a copyright notice from 1923 through 1963 without copyright renewal.
All works published without a copyright notice from 1923 through 1977.
All works published without a copyright notice from 1978 through March 1, 1989, and without subsequent registration within 5 years.
You can find more information on how to find public domain works here.