Canada announces new entry requirements for nationals of visa-exempt countries
This is Electronic Travel Authorization (eTA).
It has come into play on August 1st as a voluntary program first; then next March, 2016, it becomes mandatory when travelling via commercial transfer – i.e. flights, bus or train. If you travel via a private mode (car, for example), you are exempt. US citizen are exempt, along with a number of specific categories like: the Queen of England and her heirs.
Since US is the only country where people can drive to Canada, everyone else has to come via commercial transportation – in essence, everyone. If a UK citizen goes to US via a flight to Buffalo, and he can drive to CND, he does not need to get this authorization technically. However, the border officer would ask “why one would do so” & investigate further potentially where the visitor must disclose any and all criminal records in the past, anyway.
Find out if you need an Electronic Travel Authorization (eTA) or a visitor visa: http://www.cic.gc.ca/english/visit/visas.asp