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.
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.
