Motion Picture Association
The Motion Picture Association (previously the Motion Picture Association of America) is an American trade organization representing the major film studios of the US. Its Classification and Rating Administration issues content ratings for films.

Starting in 1990, rating descriptors were added for each film's rating, describing the reasons for the rating issued.

G

Suitable for general audiences; all ages admitted.

PG

Parental guidance suggested; some material may not be suitable for children.
 : Rated PG for sci-fi action and mild language. 
 
 Star Trek II: The Wrath of Khan (The Director's Edition): Rated PG for violence and language. 
 
 
 
 
 : Rated PG for sci-fi action and some mild language. 
 : Rated PG for sci-fi action violence, mild language and sensuality.

PG-13

Parents strongly cautioned; some material may be inappropriate for children under 13. This rating was introduced in 1984.

 : Rated PG-13 for some sci-fi adventure violence. 
 : Rated PG-13 for sci-fi action violence and peril and a scene of sexual content.
 : Rated PG-13 for sci-fi action and violence, and brief sexual content.
 : Rated PG-13 for intense sequences of sci-fi action and violence. 
 : Rated PG-13 for sequences of sci-fi action and violence.

Other ratings
There are two additional ratings that are more restrictive than PG-13, but no Star Trek production has received them.

 R: Restricted; under 17 requires accompanying parent or adult guardian
 NC-17: Adults only; no one 17 and under admitted.

External links
  – official homepage
  – database of film ratings
 
Rating systems