The British Board of Film Classification is an independent organisation that exists to regulate and classify the content of films shown in cinemas and released on video.
It does this by awarding a classification certificate based on what it considers to be appropriate age limits for the film in question - and, if deemed necessary, by requesting that the film's distributor make changes to the film, usually in the form of cuts.
In most cases, these cuts are connected with material considered too extreme for a particular age category (sex, violence, drugs and swearing). Cuts are also made to ensure that films comply with the criminal law.
The BBFC (the 'C' originally stood for 'Censors') was established in 1912 by the film industry after the introduction of the 1909 Cinematograph Act, which granted local authorities sweeping powers of censorship. The Board was created in order to establish uniform standards across the country and simplify matters for distributors, who otherwise would have been faced with having to submit films to each individual local authority for vetting.
While the BBFC's classifications remain advisory with regard to cinema releases, the 1984 Video Recordings Act gave it statutory powers for the first time. Almost all UK video releases have to be examined and classified by the BBFC - arguably the strictest system of video censorship anywhere in the Western world.
Unsurprisingly, the BBFC has been a hugely controversial organisation that comes under frequent attack from two fronts. Liberals resent it for censoring films at all, authoritarians protest that it doesn't censor nearly enough, and both groups delight in highlighting inconsistencies in BBFC decision-making.
The latter, though, is an inevitable side-effect both of classification policies that take artistic merit into account, and of the BBFC constantly adjusting these policies to reflect social and cultural change.