This is a page that documents, in detail, what the MPA ratings are and why are they classified that:

The MPA ratings

Rating G.gif General Audiences: "All ages admitted."
A film in this category will usually contain nothing or practically no violence, sex or bad language.

Rating PG.gif Parental Guidance Suggested: "Some material may not be suitable for children."
Parents are advised to accompany the child when viewing because it may be unsuitable for them. Mild violence, sex or bad language. Drug use is no longer acceptable at this category, but had been previously. Very mild detail of blood if any.

Rating PG-13.gif Parents Strongly Cautioned: "Some material may be inappropriate for children under 13."
When it comes to letting under pre-teens watch a PG-13 rated movie, parents are advised to re-think about it. Infrequent strong violence and bad language, perhaps mild if frequent, moderate sex with or without a bit of mild nudity and in the case of drugs and detail of blood it should be mild.

Rating R.gif Restricted: "Under 17 requires accompanying parent or adult guardian."
A minor under age 17 needs accompaniment from their parents or an adult guardian (18 or older). Strong bloody violence, sex or bad language (in the case of nudity moderate).

Rating NC-17.gif Adults Only: "No One 17 and Under Admitted."
Previously listed as "No Children Aged 17 Or Under Admitted." Movies with this rating are clearly intended for adults.

Name and rule changes

In 1970, the M category (Mature Audiences) was renamed to GP, and renamed again to PG in 1972. The "Mature Audiences" level indicated content stronger than permitted in a G-rated movie, rather than content intended for audiences of a legally adult age. The change to PG occurred to clear up the misconception that GP meant "General Audiences" or "General Patronage" (acceptable for all ages).

Also in 1970, the age limit for the R category was raised from 16 to the current 17 age limit. This now meant that children aged 16 years old also had to be accompanied by an adult.

In September 1990, the category of X was changed to NC-17 to indicate content intended for adult audiences rather than pornography. Any film prior to this time given the X rating by the MPAA did not receive the MPAA seal approval as this rating was not officially trademarked. Films not submitted to the MPAA for a rating could be given an "X" rating by the movie studio themselves, or any other symbol or description that was not trademarked. This practice had been used by the adult entertainment industry since the 1970s, using two or three Xs to indicate/imply stronger content.

At present, an X rating is firmly associated with pornographic content. Mainstream films such as 1969's Midnight Cowboy that were given an X rating typically were re-edited to achieve an R rating or were assigned an NC-17 rating as a replacement.

In 1996, the age limit for the NC-17 category was raised from 17 to the current 18 age limit. This now meant that young adults aged 17 years old also couldn't watch NC-17 rated movies.