If you don't like cars, I guess all these car games are quite hard to tell apart. If you do, however, you'll know that you're not dealing with a single genre so much as dozens of the things.
All the best games about cars are a little bit different: Burnout is both highly evolved and utterly, utterly depraved, while Ridge Racer is a light-streaked lucid dream where you ghost around corners and send out surprisingly gentle showers of sparks, and Forza Motorsport aims for sheer breadth of simulation options. Gran Turismo? Gran Turismo is part museum and part trip to the dentist: a clinical environment where softly-spoken experts have gathered to venerate and protect anything that comes with a camshaft.