Most common flags:
yellow: danger on a part of the track, you are not allowed to overtake.
red: stop immediately, or drive very slowly to the pit, session has stopped.
black: the number of the car with it has done something not within the rules and has o come in, mostly for a stop&go.
black with orange circle: the number of the car with it has some technical problems and has to come in.
yellow with red stripes: oil on the track, slippery surface of the tarmac.
checkered flag: race has come to an end.
blue flag: a faster competitor is behind you, let him pass within three laps.
Can't think of any other.
Niels