Here's an informal set of definitions of the MPAA movie ratings used in the U.S. If you care enough, I can get the MPAA's own explanations.
G - General audiences. There should be little in such films to offend any viewer. Most frequently children's films, but some documentaries and the occasional film for adults (like Little Dorrit) gets this rating.
PG - Parental Guidance suggested. This rating suggests that the film contains some elements that parents may not want their children exposed to, such as sex, nudity, violence, profanity, or drug use. In point of fact, much of prime time U.S. TV would be rated PG, if the MPAA looked at it, so the offensiveness level of such films is not necessarily very high. In practice, G rated films are viewed as kiddie movies, so most filmmakers inject enough mature material into practically any film to ensure it will get at least a PG rating.
PG-13 - Parental Guidance suggested, with a strong suggestion that the film may be inappropriate for pre-teenagers. This rating is similar to PG, but indicates a greater degree of potentially offensive material. There is, however, no enforcement of it, just as there is no enforcement of the PG rating. If the child can reach the ticket counter to hand over his money, he'll probably be sold a ticket.
R - Restricted. No children under the age of 17 admitted without a parent or adult guardian. This rating suggests that there are strong elements of sex, violence, or (less often) profanity in a film. In practice, the restriction simply means the child can't buy his own ticket. If he can find a cooperative adult, even a stranger, who will buy the ticket for him, in he goes. In theory, it's supposed to be the parent or someone else with a strong relationship to the child.
NC-17 - No Children under 17 admitted. Regardless of the presence of a parent or adult guardian, no children under 17 are admitted to such films. In most of the U.S., many theaters won't show such films, and many newspapers won't accept advertising for such films. (Note that since the time that this was originally written, these observations no longer seem to be true. Releases in 2004, such as The Dreamers and Young Adam, seem to have had practically no difficulties in getting bookings or advertising.) As a result, few of them are released, especially by large studios. Most NC-17 rated films are either foreign films or independently made American films. In both cases, the films have relatively small, sophisticated audiences, to begin with, and typically are only shown in art houses, regardless of ratings, so they are little harmed by the rating.
There is a defunct MPAA rating called X. It meant essentially the same thing as NC-17, with the difference that, unlike NC-17 or any of the other ratings, it wasn't trademarked. The practical implication was that any film could rate itself X, without submission to the MPAA. X rapidly grew to indicate hard core sex films. A few years ago, the MPAA replaced X with NC-17, to answer critics who claimed that the stigma of the X rating was preventing legitimate adult films from being made.
The XXX rating is actually an invention of the adult film industry, rather than the MPAA. Originally, X-rated films tended to be along the lines of Midnight Cowboy and A Clockwork Orange - frank about sexuality and definitely made for adult audiences, but not sexually explicit. Also, there was a market for nudie films at that time. These films featured lots of (mostly female) nudity and perhaps simulated humping, but no extremely explicit material. Those, too, were rated X. The adult film industry coined the XXX rating to let you know that the film in question featured closeups of genitalia in action. To the best of my knowledge, no film has ever been given a XX rating, whatever that might mean.
The PG rating started life as the M rating, then transformed into the GP rating, then switched to PG after a few years, for no particularly good reason, in either case.
The PG-13 rating was added after protests about the content of some PG rated films, notably Indiana Jones and the Temple of Doom.
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