Starting 15 MAR 2016, visa-exempt foreign nationals who fly to or transit through Canada will need an Electronic Travel Authorization (eTA). Exceptions include U.S. citizens and travellers with an existing visa.
The system is similar to the ESTA (Electronic System for Travel Authorization) currently in use by the USA. Until now, visa-exempt foreign nationals seeking entry to Canada are not systematically screened for admissibility until they arrive at a Canadian port of entry.
The Canadian pre-approval system will only be required for TRV-exempt individuals seeking to enter Canada by air to visit on a temporary basis. A fee of CAD $7.00 will be required for processing. Electronic travel authorization will be valid for a period of five years.
A number of exemptions from the requirement to obtain pre-approval to travel will be in place, including:
nationals of the United States,
individuals already in possession of a Canadian temporary resident visa,
certain foreign diplomats,
commercial air crew
individuals in possession of a visa to enter the United States on a flight bound for that country in transit through Canada, where the sole purpose of the flight stopping in Canada is for purpose of refuelling,
individuals transiting through Canada as a passenger on a flight who are in possession of any visa required to enter the country of destination
For the full details, and to apply for an eTA if you need one, take a look at the official Canada eTA site.