The British Board of Film Classification

The British Board of Film Classification is responsible for giving every new film in Britain a rating, for example a U/PG or a 18.

U stands for universal - suitable for anyone to watch.
PG stands for Parental Guidance - suitable for most people, but younger children may need to watch the film with parents because, for example, of mild comedy violence.
12 is literally the minimum recommended age for a person to watch the film. Although people under the age of 12 will be allowed to watch this film in the cinema, they must be accompanied by an adult.
15 is also the minimum recommended age for a person to watch the film, as it may contain mild nudity, violence and swearing.
18 is the highest rating a film can be given, and will probably contain frequent and graphic violence, nudity and frequent swearing.
Some films are too explicit to be given a rating and are therefore banned by the board to be shown in Britain.