Public domain works are those on which copyright has expired or which the creator or copyright owner has chosen to release as public domain.
According to the Copyright Act C-42:
Term of Copyright
6. The term for which copyright shall subsist shall, except as otherwise expressly provided by this Act, be the life of the author, the remainder of the calendar year in which the author dies, and a period of fifty years following the end of that calendar year.
For example, Robert Service died September 11, 1958, so copyright on his works expired in Canada on December 31, 2008. There are a number of exceptions, however.
Note that materials in the public domain may still contain copyright material, e.g., if an editor has added an introduction, notes, or commentary. In such cases, the material in the public domain may be copied freely, but the editorial material may not be copied beyond what is permitted by fair dealing.
Note also that every edition, translation, or adaptation of a work has its own copyright. For example, a translation of a work by Balzac would enter the public domain in Canada 50 years after the death of the translator.