The MPAA is the Motion Piction Association of America. They rate films, among other things. For those that don't know, a "R" film can get away with almost anything, and any minor with a guardian's permission can go see it. But some movies get rated NC-17 and no one under 17 or under can see them. Sometimes it's the violence that gets a film rated N-17, but you could machine gun a bus full of handicapped orphans and still get an R. More often it's the sex that gets a film rated NC-17.
Offenses include what the MPAA deem to be abberant material. This has included sexual positions other than missionary, male frontal nudity, homosexual sexuality (such as two girls in bed, under the covers with no nudity), female sexuality (such as showing a woman's face during orgasm, especially if its a long one), fetishes, and the list goes on. Yet a teenager can be bare-assed on the counter fucking a pie (American Pie), and that's R.
I like the idea of having a rating system, but I think the disparities between an R and an NC-17 ought to be addressed. An NC-17 rating is a boc office killer, even if the theatres will carry it. Most people don't understand the rating. And a parent ought to be able to alow their child to see whatever they deem appropriate, not the MPAA. I also think its no coincidence that what the MPAA considers "immoral" is so often some of the best films being produced. Granted, the system is voluntary, but theatre chains won't carry an unrated film. This is a form of censorship.