Green Flag
The Green Flag is displayed to indicate normal racing conditions and the course is open.
Yellow Flag/Start Flag
The Yellow Flag is displayed from the rescue boat at 60 seconds to the start of the race. Anytime between 20 - 0 seconds the flag will drop and this signals to the teams the the race has started.
Red Flag
The Red Flag, often accompanied by a signal flare or series of flares, automatically stops the race. 
White Flag
The White Flag is displayed to indicate the start of the last lap for the leader and their crew. The white flag is flown while the lead boat starts the last lap.
Checkered Flag
The Checkered Flag is waved to the lead boat at the conclusion of the last lap to display that their boat has crossed over the finish line first.  It is handed to the driver whenever possible to fly from the boat during the victory lap.
Black Flag
The Black Flag is displayed to close the course, when this flag appears all boats must return to either the starting dock or the pit area.