The British Board of Film Classification (BBFC) classifies films in the UK by age group. They watch each film and consider factors like violence, sex, language, nudity, mature themes, horror, and discrimination based on guidelines. These factors help them determine which age rating is appropriate. For example, a film may get a 12 rating if it contains some violence but no sex, nudity or mature themes. The document also discusses how the author's film opening would likely receive a 12 rating for having one mild violent scene but no harsh language, sex, nudity or horror.