A single entry visa allows foreign nationals to enter Canada for one-time only.
A multiple entry visa allows foreign nationals to enter and leave Canada as often as they want as long as the visa is valid.
It is not for you to choose with kind of these two visas you should apply for. There is an automatic consideration of the applicants for multiple entry visas and the applicants are only issued single entry visas under unique circumstances.
Multiple entry visitor visas allow the visa holder to travel to Canada for six months at a time and as often as they want, given that their visa remains valid. The validity of a multiple entry visa can be for up to 10 years. However, the exact validity period for your visa will be at the discretion of the visa officer issuing it. If you have a study permit or a work permit, and are not coming from a visa-exempt country, you will automatically be issued a visitor visa allowing you to enter Canada to receive your permit. Usually, this is a multiple entry visa. If, under any circumstances, you happen to leave Canada temporarily during the course of your studies or temporary work, there will be no need for you to apply for a new visitor visa to re-enter Canada. However, your visa and permit should still be valid at the time of re-entering Canada.