As of 15 March 2016, travelers will need an Electronic Travel Authorization (eTA) to board a flight to Canada.
The eTA is a new requirement for visa-exempt foreign nationals travelling to Canada by air. Much like its name suggests, it is a confirmation of a foreign national’s eligibility to travel to Canada. The eTA is electronically linked to the individual’s passport and is valid for five years or until their passport expires, whichever comes first.
Starting 1 August 2015, CIC will start accepting applications for eTA from people who wish to travel to Canada by air. This enrollment period aims to give travelers time to learn about eTA and to obtain their authorization before it becomes mandatory in March 2016.
In the vast majority of cases, these applications will be approved within minutes; however, some applicants may be found to be inadmissible to Canada and have their eTA applications denied due to existing immigration admissibility criteria, such as a criminal record or no-fly orders.
Note that U.S citizens do not need an eTA; however U.S permanent residents need to get an eTA to fly to Canada.