are two different things. when you are issued a study 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.But if you leave Canada and want to re-enter Canada, you have to apply the visa before you enter Canada even it you have a valid study 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.