The public domain includes all works that are not protected by copyright. Generally, works enter the public domain when copyright expires. The term of copyright in Canada is usually life of the author plus 50 years for textual works. Works can also enter the public domain when the creator surrenders all rights in the work to the public domain. When something is in the public domain it can be photocopied or used in any way without asking for permission. Works of the United States Government and various other governments are excluded from copyright law and may therefore be considered to be in the public domain in their respective countries.
For more information about Copyright Term and the Public Domain in the United States, see http://www.copyright.cornell.edu/resources/docs/copyrightterm.pdf