Starting March 15, 2016, Immigration, Refugees, and Citizenship Canada (IRCC) expects all visa-exempt travelers to obtain an eTA prior to entering Canada.
In the past, visa-exempt travelers did not require pre-registration prior to visiting Canada. However, as of March 15th, 2016, an online electronic Travel Authorization (eTA) application must be made prior to travel by commercial carrier. While IRCC will not enforce this change until November 10, 2016, it’s best to get your eTAs now. Trips by land or sea in a private vehicle or vessel continue to be exempt. And the eTA requirement does not apply for U.S. citizens, they can still travel as per usual.
The eTA requirement applies to employees on work permits too, who previously traveled without any additional requirements. Work permits issued after July 31, 2015 automatically included an eTA. You can check the IRCC page for full details here.