This article is from the Formula One Motor Racing FAQ, by mitchmcc@ultranet.com (Mitchell McCann) with numerous contributions by others.
Yellow - caution; no overtaking/safety car out.
Yellow with red stripes - track is slippery (usually oil).
Blue - There is considerable controversy regarding the exact use
of blue flags. This is because the instructions given to the
marshals contradict the International Sporting Code. This
definition is based on written instructions from the clerk of
the course to marshals at at least two GPs this season.
During Practice
Stationary: A faster car is catching you. Give way.
Waved: A faster car is about to overtake you. Give way
urgently.
During the Race
Stationary: You are about to be lapped. Let the other car
through.
Waved: Let the other car through immediately. You are now
risking a penalty.
(NB: The International Sporting Code states that a blue flag may
be used to allow a faster car to overtake for position. However,
its use in races appears to be almost exclusively limited to
situations where a driver is being lapped).
Red - race stopped, slow down and return to pit lane.
Chequered - race finished.
Black, with a car number - car must return to pit lane within 3
laps and not restart race (this may mean a terminal rule
infringement, but it can also mean that there is something
dangerously wrong with the car that the driver does not know
about - hence it is grossly negligent to ignore this flag)
 
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