Canada welcomes quite 35 million temporary residents (non-immigrants) annually. aside from Canadian citizens and permanent residents, all other individuals require permission to enter Canada as a visitor and need either a short-lived Resident Visa (TRV) or an ETA (Electronic Travel Authorization), unless they're US Citizens.
The following foreigners need either a TRV or an ETA:
Temporary foreign workers (individuals with work permits)
International students (individuals on study permits)
What is an ETA (Electronic Travel Authorization)?
As of March 15, 2016, travelers with passports from countries that are visa-exempt who enter Canada by air will need an ETA.
The authorization is electronically linked to your passport and is valid for five years or until your passport expires, whichever comes first.
What are a TRV and the way to use one?
The TRV may be a document issued by a Canadian Immigration Visa Office outside Canada, showing that the holder has satisfied the necessities for admission to Canada as a visitor. TRVs are also for single-entry bookkeeping or multiple entries. As a general rule, tourists are admitted for six months. Temporary foreign workers and international students are admitted for varying periods of your time, as determined on a case-by-case basis. Extensions could also be applied for Canada.
It is important to notice that possession of a legitimate TRV doesn't necessarily mean that the Officer at the Canadian Port of Entry will admit the visitor into Canada. At the Port of Entry, all visitors must demonstrate that the aim of their visit to Canada is brief. Officers at the Port of Entry will deny admission to all or any persons who, in their opinion, don't shall leave Canada at the expiry of their visitor status.