Which visa you need to enter Canada depends on what country you are from and what you plan to do in Canada. Canada does not require tourists from certain countries to get a visa, and some business visitors may not need a visa. However, students studying in Canada and most people intending to work in Canada do need to get a visa.
Tourists from the U.S. do not need a visa to enter Canada, though they do need to show the border official a valid U.S. passport. However, citizens from most other countries--a full list can be found at http://www.cic.gc.ca/english/visit/visas.asp--will need to obtain a visa from a Canadian diplomatic mission before entering Canada.
Some students can enter Canada with a tourist visa (for example, students studying for a small amount of time at a language school), which is faster and cheaper to obtain but has fewer benefits; check with your school to see if this is an option. All students, regardless of citizenship, taking courses at accredited colleges and high schools must enter Canada with a student visa, which can be obtained at a Canadian consulate or embassy in the student's country of residence.
Temporary Work Visa
If you are working temporarily in Canada (for example, as a musician on tour, a lecturer at a conference or an athlete at a tournament), you may not need a work visa. Check on the Citizenship and Immigration Canada (CIC) website for a full list of exemptions. If you want to work permanently in Canada, you must already have a job offer from a Canadian business. Businesspeople, IT workers and live-in caregivers must go through a different process specific to their professions; more information can be found on the "Working Temporarily in Canada" page of the CIC website.