The different classifications - or "ratings" are as follows:
G - General exhibition. Anyone can watch/play something with a G rating.
PG - Parental Guidance recommended. It is recommended that any person watching/playing something under the age of 15 should have a person over 18 supervising, if the content has a PG rating.
M15+ - Mature. It is recommended for people 15 years or older. Although people under 15 are allowed to watch/play it, it's not recommended.
MA15+ - Restricted to persons 15 years or older. Nobody under 15 is allowed to watch/play content with an MA15+ rating.
R18+ - Restricted to persons 15 years or older. No body under the age of 18 is allowed to watch content with an R rating. There is no R rating for computer games.
X18+ - A rare rating that is only ever used for pornographic films. No games can have an X18+ rating. It is illegal for someone under the age of 18 to view videos with an X rating.
Due to the fact that there is no R rating for games, many adult-oriented games have been refused classification by the OFLC and some have had to have content removed to fit the MA15+ rating.
Some games that have been refused classification include: Postal, Postal 2, 50 Cent: Bulletproof , Grand Theft Auto: San Andreas (originally it had an MA15+ rating, but after the "Hot Coffee" scandal it was refused classification and later re-released) and Manhunt.