Visa - the counterfoil on your passport - is an entry document and is used to travel into the country. A multiple-entry visa allows you to travel in and out of the country within the validity period of the visa. A single-entry visa is affords you only one entry; the visa is then used and invalidated.
Work Permit - is a status document authorizing work and temporary residence in the country; this is a separate document issued at PoE upon landing of a worker or in case of renewal issued from CPC Vegreville. The validity period of the WP can extend beyond the validity period of the visa. (i.e. in some cases a visa is valid only for 6 months but a WP issued can be 1-2 years as based on the passport and/or employment contract, which ever is shorter). A WP validity typically does not extend beyond the validity of the worker's passport.
Even if you have an expired visa, but a valid WP, you have valid status and authorization to work and reside in Canada until the expiry of the WP. Note however, to travel out of Canada and to return back, you will need a valid visa to re-enter the country.
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