Short-term visitors from nearly all Western countries, except parts of Eastern Europe, normally don't require visas. Visa requirements change frequently, so check with the Canadian Immigration Centre (www.cic.gc.ca) or the Canadian embassy or consulate in your home country to see if you're exempt.
A passport and/or visa does not guarantee entry. Proof of sufficient funds or possibly a return ticket out of the country may be required. Visitors with medical conditions might be refused entry if they're deemed liable to place a financial burden on Canadian health and social services (ie they admit to needing treatment during their stay in Canada).
If you are refused entry but have a visa, you have the right of appeal at the port of entry. If you're arriving by land, the best course is simply to try again later (after a shift change) or at a different border crossing.
To/from the USA
Visitors to Canada who also plan to spend time in the USA should know that admission requirements are subject to rapid change. Under the US visa waiver program, US visas are not currently required for citizens of the EU, Australia and New Zealand for visits up to 90 days. Check with US Customs & Border Protection (www.cbp.gov) for the latest eligibility requirements. Even visitors who don't need a visa pay a US$6 entry fee at land border crossings. Be sure to check that your entry permit to Canada includes multiple entries, too.