WHAT ARE ELECTRONIC TRAVEL AUTHORIZATIONS (eTAs) AND DO I NEED ONE?
Electronic Travel Authorizations (eTAs) are essentially mini-visas for the visa exempt, except for Americans who don't need an eTA or a visa (but do need a passport).
They apply to citizens of every country of the world coming to Canada by air who don't already need a Visa, which means over 50 countries like the United Kingdom, France, or Australia.
You don't need one if you're arriving by sea or land (neither means being all that common or even possible unless you're coming from the United States).
WHEN DID eTAs COME INTO FORCE?
If you're wondering why you were able to travel to Canada as recently as 2016 without the hassle of an eTA, and now suddenly find yourself surprised by being forced to complete one prior to heading out tot the airport to board your Canada-bound plane, that's because the requirement for them only came into effect in November 2016.