I've always been curious about this myself - we have a slightly different ratings system in England; mostly different labels, but there could be some deeper difference that I'm just failing to understand. Our ratings.
Uc: Universal, suitable for all (especially children)
U: Universal, suitable for all
PG: Parental Guidance
12A: Over 12s allowed, under 12s must have parental guidance
12: Over 12s allowed only (WTF is the everyday difference from the last one? :weird: )
15: Over 15s allowed only
18: Over 18s allowed only
R18: Over 18s allowed only, only available through specially licensed adult video stores (usu. hardcore porn)
I don't know how these compare to U.S. ratings though. It'd be interesting to see if material is rated the same (or as near as) on both systems.
EDIT:
www.bbfc.co.uk