An Electronic Travel Authorization or eTA allows citizens of the 55 visa exempt countries to apply online for an approval to visit Canada.
Travelers from visa exempt countries are not required to apply for a visa at a Canadian embassy or consulate but are required to apply for an eTA in order to enter Canada.
Travelers from countries that require visas are not allowed to apply for an eTA (excluding U.S. Permanent Residents) and must apply for a regular visitor visa at a Canadian embassy, consulate or visa processing office.