G: Everyone can see it
PG: Everyone can see it, might want to go with your kids
R: Kids shouldn't see this, but it's up to you
G: This is a Disney movie
PG: This is a movie your older kids want to see.
PG-13: This is a movie your older kids want to see and the bare minimum of what would be funny to an adult
R: This movie has action
X: Vintage bewbies
IMHO, the introduction of the PG-13 rating in 1984 shifted the PG rating closer to G.
Indeed... I would amend Squiggy's chart above to
G: This is for kids up to age five.
PG: this is a Disney/ family movie.
PG-13: This is 90% of all movies you will see, and runs the gambit from family fare to dirty comedy, to hard core action.
R: Lots of blood, excessive cursing, male nudity, the whole thing is about sex.
NC-17: art house sex movie, European sex movie, anything dealing with a graphic and frank look at homosexuality, excessive male nudity, political rating for a movie the MPAA did not like.