As of March 15, 2016, visa-exempt foreign nationals who fly to or transit through Canada are expected to have an Electronic Travel Authorization (eTA). Exceptions include U.S. citizens and travellers with a valid Canadian visa. However, from March 15, 2016 until fall 2016, travellers who do not have an eTA can board their flight, as long as they have appropriate travel documents, such as a valid passport. Find answers to your questions about the leniency period.
Entry requirements for other methods of travel (land, sea) have not changed.