The symbol is placed on games that have been rated by the Entertainment Software Rating Board (ESRB). The ESRB was established in September 1994 by the Entertainment Software Association as a means of regulating the video game industry. Companies voluntarily submit their games for review to determine the appropriate age group based on the content and nature of each game.
Currently, the ESRB has five ratings for games:
Early Childhood - Games with the EC mark are geared towards children ages 3 and up.
Everyone - Games marked E are well suited for a general audience. They have minimal violence but may contain some crude language.
Teens - Games with the T mark are for older kids, ages 13 and up. These games often have violent content and can contain strong language.
Mature - Games marked M are for people 17 and older. They usually have very violent or gory content, strong language and possible nudity.
Adults Only - Games with the AO mark are not suitable for anyone under the age of 18. They may contain graphic violence, language or sex. To date, no game has been published with this rating.
Games that have not yet been rated but have been approved will have the letters RP in the box for Rating Pending. An older rating that you may still find on some games is the K-A rating. This stands for Kids to Adults and was replaced in 1998 by the Everyone rating, but is essentially the same.
Here are some interesting links: