In Canada, movie ratings are a provincial designation, so movie ratings would vary from province to province. In Ontario, movies are rated as follows:
G- All audiences (formerly F, or Family)
PG- Parental Guidance
14A- Persons under 14 must be accompanied by a guardian
18A- Persons under 18 must be accompanied by a guardian
Until a few years ago, 14A and 18A were both one rating, AA. However, many movies rated AA were actually too steamy and/or violent for 14-17 year olds, which was why it was split to 14A and 18A.
R- No one under 18 is admitted
Detailed information can be found here:
http://www.ofrb.gov.on.ca/english/page6.htm