The bbfc are a group of people who give out certificates allowing a movie to be not only shown but to show the age rating.
There are seven age ratings : u, pg, 12a, 12, 15, 18, R18
U: nemo= this is a 'u' because it contains mild threat when the sharks hold nemo captive
pg:toy story=mild violence, scary scences , dangerous acts when they run into the road when there are oncoming cars
12a=contains infrequent strong language and moderate sex references
15=Contains moderate sexual references and drug references
18=Contains very strong language, strong threat and supernatural horror