After a leniency period, the requirement of having obtained an Electronic Travel Authorization, abbreviated eTA, has become mandatory for travelers, who don’t already have a Canadian visa.
As of September 29, 2016, all non-visa travelers need to apply for and obtain an eTA prior to boarding their flight to Canada (whether as final destination or transit), somewhat comparable to the US ESTA travel authorization.
An eTA is applied for online and is usually granted within minutes after submission. Some applications may take longer and may require submitting additional, supporting documents. Travelers to Canada, who already have a Canadian visa, do not have to apply for an eTA; the authorization is solely required for travelers from so-called visa exempt countries (see: http://www.cic.gc.ca/english/visit/visas.asp).
An eTA can be used for multiple entries, and has a validity period of maximum 5 years (also depending on the expiry date of the passport on which the application was made). An eTA may allow for stays up to a maximum of 6 months in the aggregate per year, whereby the actual term of stay is determined at entry by the Canadian Border Services Agency.