Grargar: From the ESRB site:
"MATURE
Content is generally suitable for ages 17 and up. May contain intense violence, blood and gore, sexual content and/or strong language."
"ADULTS ONLY
Content suitable only for adults ages 18 and up. May include prolonged scenes of intense violence, graphic sexual content and/or gambling with real currency."
In practical terms, a game that receives a Mature rating can still be sold on consoles or one of the big retailers, while a game with an AO rating will be refused release on consoles and won't be stocked by the big retailers.
Ah, interesting thanks. So AO rated games are legally forbidden to be released on consoles and can only be released on PCs? Sounds weird!