Your Career consists of 9 stages with 10 races in each. The further you are in the stage, the more robust the race will be. You might want to practice a couple of times first and then tune your car accordingly for a specific race to beat it. Every stage also has its own boss. After beating those bosses, you get special prizes!
By beating stages, you also unlock licenses which will allow you to purchase higher-tier cars from the Car Dealer.