The Genocide (G) flag fires a special shot that will kill all of the teammates of the tank that was shot; all of the enemy team's deaths are attributed to the holder of the Genocide flag.
Due to its power, it's common for Genocide flags to have a limited number of shots so it's wise to refrain from shooting unnecessarily to avoid wasting any shots. Tanks who are falling without the Wings flag flag are helpless targets as they cannot avoid landing on well timed Genocide shots.
The only way to defend against Genocide is to dodge the bullets or to use the Shield (SH) flag to avoid the affects. However, if a teammate is shot with Genocide, there is no way to negate the affect of the flag.