I believe the difference, at least in theory, is that every other rating (E through M) allows for parental discretion. So as a parent if you think your 14 yr old can handle an M game, or your 10 yr old can handle a T game you can buy it for them. It's kinda like the difference between R and NC-17 movies. Kids can see an R movie with a parent, NC-17 means no one under 17 is allowed in the theater. Now, I don't know if it's actually specified that stores shouldn't sell an AO game to a person if it seems like they're buying it for a kid, especially since AO games are so rare and many places don't even carry them (same with NC-17 movies but at least the MPAA is pretty straightforward with their industry regulations and what they mean, how they're enforced). Video games are treated in a more relaxed manner and the video game industry-retail partnership with regards to enforcing industry regulations is a lot looser. Bottom line is that in a practical, actual sense, AO is a more seriously emphasized Mature rating, reserved for things in a category of their own (explicitly pornographic, sex stuff) or that cross some sort of line with people (Hatred).