U stands for 'Universal', meaning the film, video or DVD is suitable for anyone. Examples include Happy Feet 2 and Arthur Christmasand Puss in Boots.
PG stands for 'Parental Guidance', which means that some parts of the film might not be ideal for younger children. Parents ought to think carefully before letting them watch. An examples of a PG films is The Adventures of Tintin: The Secret
of the Unicorn.
12 is now only ever seen on videos and DVDs. As with the 12A, it tells parents that the content is best suited for those aged 12 and above. Examples, both of which were certified 12A for cinema, include Iron Man 2 and Avatar.
12A is only for films released at the cinema. It tells parents that the film is best suited for those aged 12 and above. Parents can take along under 12s if they choose, so long as they go with them! Examples include Twilight - Breaking Dawn - Part 1 and Reel
Steel.
15 means that the film, video or DVD is really only suitable for those aged 15 or above. Only people aged 15 or older are allowed to view a 15 rated film at the cinema or buy or rent a 15 rated DVD, video or game.
18 means that the film, video or DVD is only suitable for adults (people aged eighteen or over). Only adults are allowed to view an 18 rated film at the cinema or rent or buy an 18 rated video, DVD or game.