Classification

The BBFC (British Board of Film Classification) sets a classification on films. They are rated on the basis of how appropriate the film is for what age group. 

The 'U' means UNIVERSAL meaning it is appropriate for all ages. An example of this would be 'Ice Age' 

The 'PG' is PARENTAL GUIDANCE. This means that the an adult must accompany the person to watch the film. An example would be 'Facing the Giants'

'12' strictly allows only those over the age of 12 such as 'Twilight'

Unless it is '12A' this would certify that only over 12's can view the film accompanied by an adult. Example 'Romantics Anonymous'

The '15' doesn't allow under 15's to view the film. Such as 'Bad Teacher'

The '18' is the same way. Example 'Inbetweeners movie'

'R18' films are only allowed in special licensed cinemas as it contains inappropriate works only adults can view. Example 'The Human Centipede 2 Full Sequence' which was banned.