UK film certificate
introduced in September 2002 by the bbfc
. It applies to cinema
films only, video
releases are still rated 12
Wheras previously, with 12-certificate films, you had to be over 12 years of age to get in to see the film, the rule for 12A films is that you can get in if you're under 12 as long as you're with someone who is over 18. This doesn't have to be a parent, it can be anybody, and they can't just buy the ticket, they have to sit and watch the film with the child.
The BBFC have been researching the idea of this certificate for a while now, seemingly due to films like Jurassic Park and Lord of the Rings, which were given PG ratings but accompanied by the warning "some scenes may not be suitable for under-8s". The film that caused them to go ahead with the plan was Spiderman. Many people complained about the 12 rating because it meant they couldn't take their kids to see it. Some county council's lowered the certificate to a PG (which they are legally entitled to do) so that they could.
The 12A certificate is also the reason why lots of film posters these days have little notes next to the certificate saying why it's been given that rating (i.e so parents can make an informed decision on whether to let their kids watch it or not), though I'm not sure why they're cropping up on 15-rated films since there are no plans for any sort of "15A" or "18A" certificates.
There's a FAQ about the 12A available on the bbfc website (it's a bit long to post here I think) at http://www.bbfc.co.uk