Green Flag - Indicates that the race is under way, and all is good.

Blue Flag with Yellow Stripe - Indicates that a lead lap car is approaching and the car being flagged is to give way, hold his/her line.

Yellow Flag -  It tells drivers to slow down and hold current positions until further notice. The yellow flag indicates that there has been an accident, track has been partially blocked or is unsafe. The yellow flag will remain out until the officials have determined that it is safe to continue the race.

Red Flag - The red flag indicates that all drivers must come to a safe and immediate stop. No driver should exit their vehicle at this time. This condition allows officials and emergency personnel to react safely to the current situation.

Black Flag - This flag tells the driver to pull into the pits on the next lap. It is often called the disqualification flag, but it doesn't necessarily mean disqualification. An official may only need to consult with the driver for a variety of reasons (loose part, fluid leak, etc.)

Crossed Green & White Flags -  The crossed flags indicate that the race has reached the halfway point.  

Crossed
Green & White

White Flag -   Indicates there is one lap remaining in the race, It's the final lap.

Checkered Flag -  This flag indicates the race has been completed. The driver that crosses the finish line first is considered the winner.