The Government of Canada implemented a travel regulation which is required for some foreign nationals to obtain before transiting or entering Canada as visitors. This travel regulation is the Electronic Travel Authorization (ETA). Essentially, it is an online pre-screening of visitors to Canada which helps catching those who may pose a security risk.
Over 50 countries who could previously travel to Canada by air without a visa are affected. If you have dual citizenship, it will now be mandatory to travel with your Canadian passport.
Applying for an ETA can be done online, approved within minutes and costs $7CAN. However, some applications may take a few days if you are asked to submit additional documents.
Visa-exempt travelers do not need an ETA when arriving in Canada by land or sea. An ETA can be valid for up to five years or until the date of your passport expiry.
Once you are approved, the document will be automatically linked to your passport, it is not a visa or a stamp on your passport. Be sure to fill out your application diligently to avoid any refusals which may cause delays in your travels.
Here is the link to apply for the ETA: http://www.cic.gc.ca/english/visit/eta-start.asp
You will need a valid passport and credit card in order to apply online.