THE OFFICIAL WESITE
MPAA Rating Descriptions
PG Parental Guidance Suggested - Some material may not be suitable for children. Signifies that the film rated may contain some material parents might not like to expose to their young children - material that will clearly need to be examined or inquired about before children are allowed to attend the film. Explicit sex scenes and scenes of drug use are absent; nudity, if present, is seen only briefly, horror and violence do not exceed moderate levels.
PG-13 Parents Strongly Cautioned - Some material may be inappropriate for children under 13. Signifies that the film rated may be inappropriate for pre-teens. Parents should be especially careful about letting their younger children attend. Rough or persistent violence is absent; sexually-oriented-nudity is generally absent; some scenes of drug use may be seen; some use of one of the harsher sexually-derived words may be heard.
This information is copied from the NATO (National Association of Theater Owners) website and this text is a direct link to their page which has more information available
G General Audiences - All ages admitted. Signifies that the film rated contains nothing most parents will consider offensive for even their youngest children to see or hear. Nudity, sex scenes, and scenes of drug use are absent; violence is minimal; snippets of dialogue may go beyond polite conversation but do not go beyond common everyday expressions.
R Restricted - Under 17 required accompanying parent or adult guardian (age varies in some jurisdictions). Signifies that the rating board has concluded that the film rated may contain some adult material. Parents are urged to learn more about the film before taking their children to see it. An R may be assigned due to, among other things, a film's use of language, theme, violence, sex or its portrayal of drug use.
NC-17 No One 17 and Under Admitted. Signifies that the rating board believes that most American parents would feel that the film is patently adult and that children age 17 and under should not be admitted to it. The film may contain explicit sex scenes, an accumulation of sexually-oriented language, and/or scenes of excessive violence. The NC-17 designation does not, however, signify that the rated film is obscene or pornographic in terms of sex, language or violence.