Electronic Travel Authorizations (eTAs) are essentially mini-visas to enter Canada 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, Australia or German.

You don't need one if you're arriving by sea or land (however neither of those means of entry is all that common or even possible unless you're coming from the United States). 

WHEN DID the eTA for canada 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 to the airport to board your Canada-bound plane, that's because the requirement for them only came into effect in November 2016.

Do I need to Complete One Every Time I Travel to Canada?

No. An eTA is good for five years of entry. Because of the number of problems we are seeing people experience in the court of trying to obtain them - some of which stem simply from IRCC's online computing systems crashing - we recommend you apply of an eTA prior to booking any travel, and if you have already booked you should at least apply as soon as possible as time to approval could be measured between days and weeks, even though in theory for most people the system is supposed to approve within minutes.