A single entry visa allows one entry only to Canada, during the validity of the visa. A multiple entry visa allows several entries to Canada, during the validity of the visa.
A transit visa allows entry to Canada for a short stop-over (maximum 48 hours) en route to another country.
If your return travel is also through Canada, you should request a transit visa valid for 2 entries.
IMPORTANT : The validity of a temporary resident visa can never exceed the validity of the passport.