The BBFC

The BBFC (British Board of Film Classification) which was first called British Board of Film Censors is responsible for the national classification of films in the UK. This non-governmental organisation is funded by the film industry. Under the Video Recordings Act of 2010, the BBFC is expected to classify DVDs, videos and certain video games.
The films and video games that are rated receive a certificate, for example '15' as well as advice for consumers pointing out whether any sex, coarse language, or violence is involved. This is used so that the appropriate certificate is assigned to a film and only people over that particular age can buy that film. 

The BBFC also advise what cuts should be made in a film. Cuts are compulsory if whats involved violates the Protection of Children Act (1978) or Cinematograph Films (Animals) Act (1937). Cuts can also be advised to make the film more appropriate for a wider audience. Also if distributors have asked for a certain certificate the BBFC may suggest cuts so that their film will fit the certificate they requested. 

These are the current certificates:

Universal - This means that all ages are allowed to watch. There is nothing unsuitable for children over four years of age.

PG - This means that all children are admitted, however some scenes may be unsuitable for children under the age of eight.

12A - This means that children under the age of twelve may only watch the film if they are accompanied by an adult. It is not recommended that children under the age of twelve should watch it as 12A films can include mature themes, moderate violence and sex, commonly used milder swear words, soft drugs and discrimination. 

12 - This means that only people twelve and above can buy or rent films or games under this certificate. 12 films can include infrequent use of strong language, infrequent drugs, moderate violence, discreet sexual activity and brief nudity.

15 - To buy a 15 rated film or game you must be over the age of fifteen as it can contain adult themes, strong language, moderate-strong violence, moderate-strong sex references, hard drugs and mild non detailed sex activity.

18 - For films of this certificate you must be eighteen or above. 18 certificate films have no limitation on bad language. They also allow hard drugs and strong violence. Strong sexual activity is allowed as well as sex references and scenes of strong real sex as long as it is justified by the context.

 R18 - For cinemas to beable to show R18 rated films they must have a license, just like retailers must have licences to sell the film. Only people eighteen and over may buy these films. These films include all whats in an 18 rated film however the violence and sex activity would be stronger. These films get demanded cuts more than any other certificate films.