It is based upon the FIA GT series. They have almost every car that competed in the series in 2003 and 2004, minus a livery or two from privateers. Every track that was raced in the series appears as well. As others will attest, there are some really fun tracks to run. Plus realistic simulation of dynamic weather and time of day.
The cars off the top of my head:
Corvette C5R
Dodge Viper Competition Coupe and variants
Ferrari 360 Modena and variants
Porsche 911 GT3 cup car and variants
BMW M3 GTR
BMW Z3 M Coupe
Nissan 350Z
TVR something 400
some Seat that I can’t remember
Lotus Elise
Saleen S7
…and a couple others that Rick mentioned before I was done typing.