Canada has introduced a new entry requirement, known as an Electronic Travel Authorization or eTA, for visa-exempt foreign nationals travelling to Canada by air. Exceptions include U.S. citizens and travellers with a valid visa. Entry requirements for other methods of travel (land, sea) have not changed. Starting August 1, 2015, eligible travellers can apply online for an eTA. On March 15, 2016, this entry requirement will become mandatory and travellers will need an eTA before they can board a flight to Canada.
For more information, see http://www.cic.gc.ca/english/visit/eta.asp.