Not required for citizens of Australia, New Zealand, United Kingdom and the United States, among others. See www.cic.gc.ca.
Citizens of dozens of countries – including the USA, most Western European countries, Australia, Japan and New Zealand – don’t need visas to enter Canada for stays of up to 180 days. US permanent residents are also exempt. Note that you still need an Electronic Travel Authorization (eTA) to fly into Canada. The eTA costs $7 and you can apply for one online at www.cic.gc.ca/english/visit/eta-start.asp; the approval proccess usually only takes a few minutes. US citizens and permanent residents do not require eTAs.
Nationals of around 150 other countries must apply to the Canadian visa office in their home country for a temporary resident visa (TRV). See www.cic.gc.ca for full details. Note that if you have a visa, you are not required to purchase an eTA.
Single-entry visitor visas are valid for six months, while multiple-entry visas can be used for up to 10 years, provided that no single stay exceeds six months. Either type of visa costs $100. Extensions cost the same price as the original and must be applied for at a Canadian Immigration Center one month before the current visa expires. A separate visa is required if you intend to work in Canada.
For the latest customs information, contact the Canadian embassy or consulate at home, or go to the 'Visit as a Tourist' section of the Canadian government website (www.cic.gc.ca).