Visitors to Canada can either require a TRV or be visa exempt. If you find out that you are visa exempt then you may require an eTA or Electronic Travel Authorization to travel to Canada. Some instances where eTA may or may not be required include:  

• Visa-exempt foreign nationals will need an eTA to fly to or transit through a Canadian airport. These travellers do not need an eTA when arriving by car, bus, train or boat (including a cruise ship). 
• Lawful permanent residents of the U.S. need an eTA to fly to or transit through a Canadian airport. They must present a valid Green Card and a valid passport at check-in. 
• Citizens from certain visa-required countries may be eligible to apply for an eTA, instead of a visa, to travel to Canada by air. However, these travellers need a TRV when arriving by car, bus, train, or boat, including a cruise ship. 
• U.S. citizens are exempt from the eTA requirement and must carry proper identification such as a valid U.S. passport.