A single entry visa allows for a single entry into Canada, but a multiple entry visa allows for many entries. This is the fundamental distinction between the two types of visas.
A multiple-entry permit: A multiple entry visa is good for as many trips into Canada for a period of six months as you’d like. It will be valid for up to ten years or until one month before to the expiration of your passport, whichever comes first. Prior to the expiration date on your visa, you must arrive in Canada.
Single-entry permit: The single entry visa allows you to enter Canada once only. You might only be qualified for a single entry visa, for instance.