Hmm.. where to start.
First, I am going to move this to the Autocross/Motorsports forum. There is a thread there about classing (SS or ASP).
Classes are divided into allowed preparation levels.
STOCK This is covers most factory stock cars and mods are limited to struts, front swaybar, tires, same dia/width rims, exhaust. Some misc. They are listed in classes Super Stock (SS), and then A through H. H are the lowest potential.
STREET PREPARED Stock cars that are allowed certain modifications such as increased intake, exhaust, suspension allowances. Diff wheels. Classes are listed as ASP through FSP, wtih ASP the fastest.
PREPARED Tube frame and heavily modified cars.
MOD Purpose built race cars such as open wheel and winged beasties.
OtHER