How can I know which cars are in the different classes, i.e. GT, Super GT,
GT500, Gt300, LM, LMP, etc.?
What sets them apart?
GT= Gran Tourer, or Gran Turismo. It's a high performance luxury car. Most cars has a two-door seating.
Super GT= If you mean by non-vehicular terms, it's a racing series taken place in Japan. Super GT cars are what are used in the racing series. You can tell by heavily livery-ed cars, like the Xanavi Nismo.
LM= Le Mans cars. These cars are used on the 24 Hours of Le Mans. Le Mans cars are divided based on performance. One example of an LM car is the Jaguar XJ13.
LMP= Le Mans Prototype cars. These cars are used on the 24 Hours of Le Mans & other series based on Le Mans. These cars are fastest in circuit racing, making them comparable to F1's. One good example of an LMP car is the Audi TDI.