Certification of Films

U

U stands for Universal. This means that there is almost nothing deemed unsuitable for the viewing of children, and that the viewing will be fit for audiences of all ages.

Language: Only very mid bad language (e.g damn and hell)
Sexual nature: Only kissing and cuddling is permitted
Drugs: No references
Violence: Very mild fight scenes, scenes where character's are placed in danger 

PG

PG stands for Parental Guidance. This means that the film will 

be suitable for most audiences, however some scenes may be unsuitable.

Language: There may be mild bad language (e.g shit and son of a bitch.)

Sexual nature: Very unlikely, unless they are infrequent and undefiled.

Drugs: Illegal drugs may be referenced but not shown explicitly or in detail.

Violence: Very mild. May show some blood, but generally not how the injury was inflicted.

12/12A

A film that is classified as a 12 means that the viewer has to be of at least 12 years to view the film, whereas 12A means that they can view the film in a cinema if accompanied by an adult.

Language: The BFFC's guidelines suggest that strong language, such as 'f***' may be deemed suitable to appear in 12/12A films, but this is dependent on its delivery, the character using it, and the context in which it's delivered.
Sexual Nature:  Sex may be discussed or shown briefly, but not to an extent that would be unsuitable for young teenagers. Nudity may appear, but not in a sexual context.
Drugs: There may be an appearance of illegal drugs, but they will usually be portrayed negatively and will not be glamorised/detailed.
Violence: There may be some mildly violent scenes, although it will not be dwelled upon and will only be showed if it's contextually justified. Accessible weapons such as knives used in fight scenes will not be glamorised.

15

A film that is rated at 15 is considered unsuitable for viewing for anyone younger than 15 years old.

Language: Frequent use of strong language. There is no limit to the amount of strong language used (e.g 'f***'). There may be use of very strong language (e.g 'c***') but this would be rare and would have to be contextually justified.
Sexual Nature: Sexual scenes can be shown but not in overly explicit detail, although scenes can often be long. There may be strong sexual references.
Drugs: Illegal drug use may be shown, but the film as a whole must make sure to discourage use and show it in a negative manner.
Violence: Violence may be very strong, however the very strongest scenes of gore and violence may not be shown.

18

An 18 rated film is considered unsuitable to be viewed by those under 18.

Language: There is no limit to the amount of strong or very strong language.
Sexual Nature: There may be strong scenes detailed scenes of sex as long as it is not a sex works. There are no constraints on nudity in a non-sexual context.
Drugs: Drug taking may be shown extensively, but again mustn't promote use.
Violence: Violence may be shown strongly (even in a sadistic nature) and injury and infliction may even be dwelled upon.