3 difficulty levels that affect how "poorly" the AI performs:
Must come in first to win the race (second or third doesn't count)
No penalty for restarting or quitting, so if things aren't going well feel free to do so
Primary winnings are stars (+1) & FUEL awarded for each difficulty level beaten the first time. (The FUEL award is an arbitrary amount based on the race, but same amount awarded each difficulty level you beat. EG: A race may award 500 FUEL per difficulty.)
You don't have to do the races in order of difficulty. You can start out on Rookie and work your way up, or you can jump right to Expert or Legendary and try your hand at those. But you only get the (EG: you can start on Legendary, and get +3 stars if you beat it).
You can re-run difficulty levels you've beaten as much as you want, but the awards are only given once.
Races usually let you pick from a class of vehicles (eg: Trucks, ATV's, Motorcycles, etc). If you're having trouble with a certain race then simply work on other races to unlock new zones (and vehicles). Come back later when you have a better vehicle.