Ok, over here, we have:
G: General audiences. Anyone, kids, adults, what have you. Usually they give kids movies this kind of rating.
PG: Parental Guidance suggested. Just what it sounds like.
PG-13: Parents recommended to come in with anyone under 13, due to slightly more mature content.
R: Restricted. Meant for those 17 or older. The wiseasses in charge here keep trying to make it so people have to have adults with them to see this. It's not enforced all that well though. Usually this rating is given to the really violent or sexy films, unless it passes even that mark, in which case it's given an...
NC-17 rating: This is the most restricting rating a film can get. This means that if you're under 18, and you want to see this movie, you're going to have to fight through the whole danged National Guard to see. Movies of this rating include Showgirls.