You can choose to race in one of twelve Series. At the start of the game, only Series 1 is unlocked. Each time you win a race, you will be awarded Flags. The number of Flags you win in each race is defined by what Series it is in. There is also a maximum number of Flags that can be collected in each Series.
Winning a race in the higher Series will award more Flags. However, when you lose a race, you will also lose Flags from your total. The number of Flags required to unlock a Series can be found on each Series panel. The entry fee for Races increases in each Series - but the amount you can win increases too.