Difference between a single and a multiple entry visa
If you need a visa to come to Canada, you can apply for a visitor visa (also known as a temporary resident visa). There are two types: a single entry visa and a multiple entry visa. Both are valid for a fixed period and cannot be used after they expire.
A multiple entry visa allows visitors to come and go from Canada for six months at a time, without having to reapply each time. It can be valid for up to 10 years, or one month before your passport expires, whichever is earlier. You must arrive in Canada on or before the expiry date on your visa.
A single entry visa allows you to come to Canada only one time. Once you have left Canada, excluding travel to the United States and St. Pierre and Miquelon, you will need a new visa to travel back to and enter Canada.
If your visa is still valid and you are travelling only and directly to the United States (including its Territories and Possessions) or St. Pierre and Miquelon, you do not need a new visitor visa to return to Canada. If your visa expires before returning to Canada from one of these areas, and you are not in possession of a valid visitor record, work permit, study permit, or temporary resident permit (authorizing re-entry), you will need to get a new visitor visa.
Starting on February 6, 2014 (00:01 EST), all visa applicants will automatically be considered for a multiple entry visa. The visa officer reviewing your application may be able to give you this type of visa even if you applied for a single entry visa.