This article is from the Motorsport FAQ, by A H Henry email@example.com with numerous contributions by others.
Blue flag with the yellow stripe (stationary): displayed to slower cars
indicating that a faster car is attempting to pass. It is not a move
over command on road courses (not sure what it means on ovals). It is
merely advisory, although drivers can be called in for not checking
Yellow flag (stationary): displayed when there is a car stopped off
line. it is also displayed at the flag station prior to waving yellows
as a backup. There is no passing allowed from the flag to the incident.
Yellow flag (waving): displayed when there is great danger such as a
car stopped on the line (or just about anywhere on the pavement at a
natural terrain road course). Again, no passing until you pass the
Double stationary yellow: displayed on road courses indicating a full
Yellow flag with red stripes (stationary) (aka surface flag, debris
flag, hazard flag or oil flag): displayed when there is oil, water,
debris, etc on the course. Anything that could create a hazard.
Normally kept up for 1 lap until everyone has seen it, then it is
Black flag (from designated black flag station and the bridge):
displayed when they want the driver to report immediately to his/her
pit. Reasons vary from rule infractions to a mechanical problem that
requires immediate attention. If the black flag is displayed from all
corner stations, the session/race is red flagged and immediately ends
until the course condition that causes the red is cleared.
White flag with a thin red cross: displayed when a safety truck is
between that station and the next station. White flag (stationary) is
displayed at the station prior to the waving white. It is also
displayed for 2 stations for a slow moving race vehicle.
White flag at the start/finish line: last lap.
Checkered flag: displayed at the end of the race.
Green flag (from the bridge): starts or restarts a race.
Green flag (from a corner station): displayed if its been waving
yellow for more than a lap.
Red flag: displayed only from the bridge to stop a session or a race.