Great graphics, loads of cars, and hours of gameplay.
When you begin your career on Forza 3, you will be asked to choose an F class car and a race tournament to take part in (If I remember correctly). Races in the beginning of your cereer will start short, around 5 miles. Once you begin stepping up your car and car class, you the races will get longer.
The game uses a system with levels (level 1 to 50). As you race and get more credits (form of money in the game), and you will gain experience which will allow you to level up.
It is a great game for fans of racing games and cars. You can easily put 100+ hours of gameplay into this game if you enjoy racing (it will probably take that long just to beat the career!).