Study permit/work permit and visiter visa are two different things. persons from certain countries need both of them(www.cic.gc.ca).When you are issued a study permit work permit, a visiter visa is issued to you at the same time. This visiter visa has two type: single-entry and multiple-entry.
if it is a single entry type, you can only use it for one time. That means as soon as you enter Canada, it expires, but you can legally stay at Canada with a valid study permit/work permit enen the visa has expired. But if you leave Canada and want to re-enter Canada, you have to apply the visa before you enter Canada even you have a valid study permit/work permit. This visa can only be issued outside Canada.
if it is a multiple type, you can re-enter Canada as many times as you like. it has nothing to do with your study permit/work permit.
Check whether you are from the countries in which both visa and permit are needed.