Differentiating between a certificate 18 and certiface 15 film.

Certificate 15 - Suitable only for 15 years and over

No one younger than 15 may see a ‘15’ film in a cinema. No one younger than 15 may rent or buy a ‘15’ rated video work.

A certificate 15 film has a theme, however must be appropriate for the audience. Drugs may be shown in a certificate 15, however they cannot promote or encourage misuse of drugs. Frequent use of strong language is accepted, but the likelihood of stronger terms being repeated is highly unlikely, however terms which are stronger may be heard if in context. There are no constraints against nudity when it's in a non-sexual context. However, in a sexual context nudity and sexual activity may be portrayed but without strong detail. Lastly, violence may be strong, but will not dwell on the infliction of pain or injury; strong, sadistic violence is unlikely, however there may be discreet verbal references with strong contextual justification.

A certificate 18 - Suitable only for adults

No one under the age of 18 may see an 18 film in cinema nor can they rent or buy one.

A certificate 18 film entitles the over 18's to be free to choose their own entertainment. However, content appearing to promote dangerous acts to both the audience and society are deemed unacceptable. As well as this, any sexual or sexualised violence which may eroticise or endorse sexual assault does not follow the principles of a certificate 18. Lastly, explicit sexual images must be justified by the context.

http://www.bbfc.co.uk/classification/guidelines

Differences:

Clearly, a certificate 18 allows much more gore, sexual references and strong language in contrast to a certificate 18. Certificate 18 accepts far stronger language and detailed explicitness and violence. However, both certificates must be justified by the context for the allowance of explicit content.