From the web-site of the Motion Picture Association of America (http://www.mpaa.org):

THE PURPOSE OF THE RATING SYSTEM

The basic mission of the rating system is a simple one: to offer to parents some advance information about movies so that parents can decide what movies they want their children to see or not to see. The entire rostrum of the rating program rests on the assumption of responsibility by parents. If parents don't care, or if they are languid in guiding their children's moviegoing, the rating system becomes useless. Indeed, if you are 18 or over, or if you have no children, the rating system has no meaning for you. Ratings are meant for parents, no one else.

The Rating Board does not rate movies on their quality or lack of quality. That is a role left to film critics and audiences. Had we attempted to insert ourselves into judging whether a film is "good" or "bad" or "indifferent" we would have collapsed the system before it began.

The criteria that go into the mix which becomes a Rating Board judgment are theme, violence, language, nudity, sensuality, drug abuse, and other elements. Part of the rating flows from how each of these elements is treated on-screen by the filmmaker. In making their evaluation, the members of the Ratings Board do not look at snippets of film in isolation but consider the film in its entirety. The Rating Board can make its decisions only by what is seen on the screen, not by what is imagined or thought.

There is no special emphasis on any one of these elements. All are considered. All are examined before a rating is applied. Contrary to popular notion, violence is not treated more leniently than any of the other material. Indeed many films rated X in the past and NC-17 now, have at least tentatively been given the "adults only" rating because of depictions of violence. However, most of the directors/producers/distributors involved have chosen, by their decision, to edit intense violent scenes in order to receive an R rating.