When copyright has expired, or if an author chooses to waive their copyrights, works become part of the “public domain” and may be freely copied, distributed, adapted, and performed without obtaining copyright permission. Determining if a work is public domain is not always straightforward. The date a work enters the public domain depends on a number of factors including what country you are in, if the author was an individual, corporation, or government agency, if the author is known or unknown, if there are multiple authors, and if the work in questions is a photograph.
The University of Alberta has produced a Public Domain Flowchart that can help you determine if a work is considered to be public domain in Canada. Remember that even if a work is public domain, you must still provide an accurate attribution/citation.