This is a very broad question. In general, the sale or leasing
of cars without a proper disclosure of known defects or dangerous
conditions with the vehicle constitutes auto fraud.
There are various types of fraud. To name a few:
forging your signature on a document
rolling back or replacing the odometer
falsifying figures or terms on a lease
lying about the previous ownership history
One of the most common types of fraud encountered by people is
being sold a used car which later turns out to have extensive presale
Often the consumer, before purchasing the vehicle, asks the salesman
if the car was ever in an accident and the salesman replies something
like this: "Absolutely not! If it had been in an accident I
couldn't sell you this vehicle even if I wanted to." Sometime
later, when you discover to your chagrin that the car indeed had
sustained major collision damage, the salesman's false claim may
play back in your mind, but be assured that the salesman is never
going to admit to having made such a pronouncement.
Be sure to check the VIN number of a car you plan to buy beforehand
to find out for yourself that the car is not a lemon. Be aware that
even with a lemon finder, there is still a risk when buying a "as
is" used car.