There are two exceptions: you can visit the United States or Saint-Pierre et Miquelon and return to Canada without getting a new visa, as long as you:
* return within the initial period authorized by the immigration officer or
* have a valid visitor record, work permit, study permit, or a temporary resident permit (authorizing re-entry) and return within the initial period authorized by the officer
http://www.cic.gc.ca/english/information/faq/visit/visit-faq01.asp
There are two exceptions: you can visit the United States or Saint-Pierre et Miquelon and return to Canada without getting a new visa, as long as you:
* return within the initial period authorized by the immigration officer or
* have a valid visitor record, work permit, study permit, or a temporary resident permit (authorizing re-entry) and return within the initial period authorized by the officer