Some foreign nationals have to obtain a Temporary Resident Visa (TRV) to enter or to re-enter Canada as a visitor, a worker or a student. A TRV is a visa stamped in your passport which can allow you to have a single entry or multiple entries to Canada.
You can apply for a TRV before entering Canada through a Canadian a Canadian Consulate or High Commission abroad. Once you are in Canada, you can choose to extend this visitor status through an In-Canada Extension of a Visitor Status.
Canada does not require a TRV from citizens of certain countries. If you do not need a TRV, you might need to apply for an Electronic Travel Authorization (exempt US citizens). In either case, it is not guaranteed that you can enter Canada, as you might need to prove additional facts, such as your financial capacity, your ties to your home country or the reasons for your stay in Canada. If Canada Border Services Agency (CBSA) does not believe that you will depart Canada at the end of your stay or that you might engage in unauthorized work, you might be denied entry or given a date by which you have to exit Canada. You can also be extensively questioned and your profile can be flagged for future entries or immigration applications.
You might also be medically or criminally inadmissible to enter Canada, and not be eligible to apply for a TRV. In this case, you might, however, be eligible to apply for a Temporary Resident Permit.
Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay or an immigrant visa for permanent residence.