Ratings guide: (from movieratings.com)

[G] General Audience : All ages admitted. This 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.

[PG] Parental Guidance Suggested : Some material may not be suitable for children. This 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. This 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; one use of the harsher sexually derived words may be heard.

[R] Restricted-Under 17 requires accompanying parent or adult guardian : This signifies that the rating board has concluded that the film rated contains 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 : This 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, or scenes of excessive violence. The NC-17 designation does not, however, signify that the rated film is obscene or pornographic.
Credit to TwirlWriter for first responding to the question about the ratings.