Understanding how marriage records are collected and organized will make your research that much more effective. There are many websites that contain authoritative information on researching marriage records.
Since vital registration is a provincial responsibility in Canada, the most authoritative resource for Ontario marriage registrations is the Archives of Ontario. The links on the right explain how marriage registrations were collected and how to access them.
Created by the Archives of Ontario, this pathfinder clearly lays out how to find a marriage registration for an Ontario marriage, and includes other resources for obtaining these records.
This pathfinder from the Archives of Ontario (PDF format) is a handy one-page guide for locating marriage registrations in Ontario.
The Archives of Ontario microfilms and makes marriage registrations available to the public. In order to locate a record, you first need to find it in the index, which will in turn point you to the record. By consulting the table on this website, you can find the correct "reel" of the index for the marriage record you are searching for.
This webpage from Library and Archives Canada provides links for obtaining marriage registrations in Canadian provinces and territories. As marriage registrations are under provincial jurisdiction, LAC does not hold any records themselves, but this page provides links and resources for each province and territory.