Flak refers to anti-aircraft warfare, derived during World War II from the German Flugabwehrkanone, for "aircraft defence cannon". The most recognized of these guns was the feared '88', a reference to the 88mm high velocity gun that the Germans used in both aerial defense and anti-tank roles. The Germans developed a wide range of weapon systems ranging from 20mm up to 128mm. The 88mm and 105mm guns systems were first aimed using optical sights, but by 1944, the Germans had radar controlled systems which greatly improved accuracy.
In the first of the following photos you will see an 88mm gun in the background. A series of rings around the gun tube were painted on for each aircraft shot down. These "kill rings" were also applied to the gun barrels of Panther and Tiger tanks indicating the number of enemy tanks destroyed.