The rating system by the Motion Picture Association of America is very known in the USA. The "G", "PG", "PG-13", "R", and the "NC-17" classifications have been made as your guide to know what you as a parent should or shouldn't show your kids, depending on the movie. However, you should know the history of hard censorship and the corrupt business incentives that led to the MPAA, before you take the ratings too seriously.—chribren