The ESTA and the eTA are electronic travel authorisations. All travellers headed to Canada are required to apply for an electronic travel authorisation beforehand. It's important that you check before your trip to Canada which authorisation you need to apply for. Try to arrange this at minimum a week before your flight to Canada, so you won't need to make use of the urgency procedure. Both the ESTA and the eTA can be applied for online and are sent to you by e-mail.
The eTA or ESTA for Canada
ESTA is an abbreviation and stands for Electronic System for Travel Authorization. eTA stands for electronic Travel Authorization. Although these two travel authorisations strongly resemble each other, they serve completely different purposes. If you are travelling to Canada, you will need an eTA at the very least. This is issued by the Canadian government and can be applied for through this website. The ESTA is not required for Canada, unless you also plan to visit the United States during your trip. Many people mix up the eTA and the ESTA, which, considering the names, is not surprising. The ESTA is for a trip to the USA, the eTA for your trip to Canada.
ESTA required for transfer to Canada
If you are flying through the United States to Canada, you need to apply for both an ESTA as well as an eTA. Both travel authorisations can be applied for through this website: ESTA application and eTA application. You already need to possess an ESTA before getting on the plane to the USA, and the eTA is checked for when you board the second plane (after the transfer) to Canada.
ESTA required for transit through Canada?
Canada isn’t your final destination, but you’re instead flying through Canada to the USA? Then you likewise need both the eTA and ESTA. Even if you don’t leave the airport in Canada. The only exception is if you are on transit through Canada and you won’t get off the plane; for instance if the plane makes a short refueling stop. In this (fairly exceptional) case, you only need to apply for an ESTA.
There are two different forms; there is an ESTA form which you need to fill in for your authorisation to travel to the USA, and there is a different form with which to apply for the eTA to travel to Canada. Both the eTA and the ESTA are generally approved within an hour of filling in the required form. As these two authorisations aren’t physical documents, you don’t need to print them. The airline can check in the database with approved ESTAs and eTAs whether a travel authorisation was issued to you.