Canada welcomes more than 35 million temporary residents (non-immigrants) each year. Except for Canadian citizens and permanent residents, all other individuals require permission to enter Canada as a visitor.
Unless they are citizens of a visa-exempt country, individuals who wish to enter Canada for a temporary purpose, such as tourists, temporary foreign workers (individuals with work permits) and international students (individuals on study permits) must apply for and be granted a Temporary Resident Visa (TRV).
Citizens of visa-exempt countries intending to travel to Canada by air are expected to have applied for an obtained an electronic Travel Authorization (eTA) before their departure to Canada. Exceptions to this include citizens of the United States, who do not require a TRV or an electronic Travel Authorization (eTA), and Green Card holders in the United States, who need an eTA to come to Canada, regardless of their nationality. Unless otherwise exempt from the requirement to obtain a TRV or an eTA, individuals who require a TRV do not require an eTA, and vice versa.
The TRV is a document issued by a Canadian Immigration Visa Office outside Canada, showing that the holder has satisfied the requirements for admission to Canada as a visitor. TRVs may be for single entry or multiple entry. As a general rule, tourists are admitted for a period of six months. Temporary foreign workers and international students are admitted for varying periods of time, as determined on a case-by-case basis. Extensions may be applied for from within Canada.
It is important to note that possession of a valid TRV does not 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 purpose of their visit to Canada is of a temporary nature. Officers at the Port of Entry will deny admission to all persons who, in their opinion, do not intend to leave Canada at the expiry of their visitor status.
Source : canadavisa