So the races/events are split into day and night. Day races are legal, you earn money by winning those races. Night races are illegal Street races which will earn you rep. Both money and rep are important because rep unlocks new sh*t and you need money to buy them. New story missions are unlocked as you earn rep and also upgrade your car. Cars are also unlocked based on your progression. Also there seems to be various disciplines like drift, race, off-road etc but I just started and have seen only on road race and drift events.