The ESRB rating system consists of E, E10+, T, M, AO, and eC.
eC means "Early Childhood", suitable and/or intended for young children.
E means "Everyone", suitable for all ages. Replaced the K-A "Kids to Adults" rating in 1998.
E10+ means "Everyone 10+", the content is suitable for ages 10 and older. Introduced in 2005.
T means "Teen", the content is suitable for ages 13 and older. In the US and Canada, T-rated games may be sold to anyone.
M means "Mature", the content is suitable for ages 17 and older. In the US, a person under 17 may not legally purchase M-rated video games unless with parental supervision, and in Canada, under provincial laws, it is illegal to supply M-rated games to a person under the age of 17 years.
AO means "Adults Only", that no under 18+ can buy a game with that rating. So far, only 29 games have successfully kept the rating. These games usually contain lots of sexually explicit material, or in rare cases, extreme violence.
RP means "Rating Pending", that is only used in advertising and the content has not yet been rated.
An AO rating is very seldom awarded and is also a rare rating. As per above, only 29 games have successfully kept this rating, and it places massive restrictions on game sales. Because Nintendo, Sony or Microsoft will not allow AO rated games on their consoles, publishers of AO rated games usually have to stick with PC/Mac. Most games that have been successful with this rating have been released on the PC/Mac (with the exception of Grand Theft Auto: San Andreas infamous "Hot Coffee" mod that temporarily changed the rating from M to AO).