Green flag: Displayed at the start of the race, and during restarts. The lead driver cannot be passed on the track before the green flag is waving.
Yellow flag: Signals a caution, which tells drivers to slow down to a predetermined speed. Debris on the track or a wreck are typically the chief culprits for this flag.
Red flag: Signifies the race must be stopped immediately, regardless of the cars' position on the track. This is usually for safety reasons -- for example, a red flag was issued following Juan Pablo Montoya's collision with a jet dryer at the 2012 Daytona 500.
Black flag: Think of the black flag as the "consultation flag." Whichever driver is given the black flag must respond to a concern from NASCAR. Common examples include speeding on pit road, dropping debris on the track or failing to maintain the minimum speed on the track. Drivers have five laps to respond to the black flag.
Blue flag with diagonal yellow stripe: This is the courtesy flag shown to drivers not on the lead lap. It indicates that the slower drivers should yield to the faster drivers approaching them.
White flag: Shown when the lead driver begins the race's final lap.
Checkered flag: The most famous flag, the black and white checked flag is waved when the winner has crossed the start/finish line.