Here's a quick guide to the currently supported (and unsupported) game ratings. This is still a work in progress.
Currently Supported:
No Rating:
If a game has no rating... then it doesn't get a rating. You select "None" in the listbox.
Please do not use the blank option.ESRB:
The US standard. Largely unchanged since its creation, except for replacing the Kids to Adults (K-A) rating with Everyone (E).
VRC:
(Picture pending.)
Mostly used on the Sega Genesis, Sega CD and Sega Game Gear. Only two PC games to my knowledge have VRC ratings: the original releases of Mortal Kombat and Leisure Suit Larry 6. Later releases of those two games used ESRB ratings.
PEGI:
Used in Europe. This is the "redesigned" version; the original version had a + sign next to the numbers for some reason. The ratings are otherwise the same (so a Pegi 3 game is a Pegi 3+ game.)
ELSPA:
(Picture pending)
Used in some European countries. There are only four ratings: 3+, 11+, 15+ and 18+. More or less the same as PEGI, only done by different people.
Currently Unupported:
BBFC:
USK:
CERO:
The above is the current standard. Before that, the ratings were A, 12, 15 and 18.
Sega Saturn rating system (Japan):
Green = All Ages. Yellow: Some mature content. MA-18: Violent Content. X / Last image: Has nudity and adult content.
RSAC
Explained here. Has three different rating categories - Violence, Nudity / Sex and Language.
Feel free to suggest other rating systems so matt can add them later on.