Some Background on the MPAA

The MPAA is not officially a "censorship organization". It is an independent "voluntary" organization formed in the late 1960's for the purpose of "informing parents about the content of films so they can determine what movies are appropriate for their kids" (MPAA.org).
The MPAA uses a system of five possible ratings (All descriptions pulled directly from the MPAA site):

G: General Audiences. All Ages Admitted
PG: Parental Guidance Suggested. Some Material May Not Be Suitable For Children
PG-13: Parents Strongly Cautioned. Some Material May Be Inappropriate For Children Under 13
R: Restricted. Children Under 17 Require Accompanying Parent or Adult Guardian
NC-17: No One 17 and Under Admitted

Sounds awesome, right?

In some senses, it is pretty helpful. Any film rated of PG-13 and up gives a little tagline explaining why it got that rating and is a very useful tool for parents. However, the MPAA's ideas of what is appropriate for the eyes of children can be a tad arbitrary. Violence gets a much lighter touch than even the most inexplicit sexuality, and the number of times a certain swear is used can bump a PG or PG-13 to an R.

Also, while the MPAA claims their rating services to be voluntary, and a filmmaker has every right to run a film as "Unrated" if they don't agree with the MPAA's rating, most theaters will not run an unrated movie. Many theaters and video stores will also not run/sell a movie with an NC-17 rating.