It is possible to come to Canada as a visitor for reasons such as tourism, to visit family members, to attend a conference or event, or for business prospecting. Depending on your reason for visiting, you may be able to extend your stay without leaving Canada.
If you are from a visa-exempt country, and plan to travel by air, you will require an ETA to visit Canada. If you are from a visa-exempt country and will arrive by land or sea, you do not need an ETA.
If you are from a visa requiring country, you will need a visitor visa (also known as a Temporary Resident Visa) before coming to Canada.
If you are a citizen or permanent resident (green card holder) of the United States, you do not need an ETA to enter Canada.