Do they just use the American rating of G, PG, R, etc.? Or do they use some different system?
Although I have not seen a trailer for a film in Canada, it is my understanding that Canada and the US have used the ratings established by CARA - the Classification and Rating Administration - since 1968. This is confirmed at Filmratings.com
More or less, and there is some variation from province to province. I think ours differs a little; it goes like this in Ontario:
G or Family: Good for everyone.
PG
AA (No kids without adults)
R (No kids at all)
Adult (Porno)
Some provinces have split AA into 14A and 18A, e.g. no kids under these ages without adults. Quebec, inexplicably, has broken AA up into THREE categories (13A, 16A, and 18A) and has no PG rating.
I don’t see how it’s possible, every province has autonomy over their rating systems. They’re probably similar, though.