The general requirements for a Travel Visa are as follows:
Reasons for possibly being admissible to Canada include but are not limited to any criminal activity, human rights violations, organized crime, security risks as well as health and financial reasons.
Every traveller needs to be in possession of a valid passport (or unabridged birth certificate in the case of children) in order to travel across the Canadian border, no matter their country of origin. The passport needs to have an open page for immigration and passport control officers to stamp as you move across the borders or airport control points.
A lot of countries are exempt from applying for a Canada Visitor Visa and will only need an Electronic Travel Authorization (eTA) but those that are not exempt need to appear in person at the Canadian embassy in their country of residence. A biometric screening is done by an embassy consultant and the travel documents that you must collect and present are checked for approval (generally over a 4-6 week waiting period however this depends on your nationality and where you are applying from).