Short answers, pulled from the story.
Anti-aircraft warfare, also called air defence, is the counter to aerial warfare and includes all measures designed to nullify or reduce the effectiveness of hostile air action. It covers surface-based, submarine-launched, and air-based weapon systems, along with sensors, command and control arrangements, and passive measures such as barrage balloons.
The first aircraft shot down by ground-to-air artillery fire was downed on the 30th of September 1915 at Kragujevac, when Serbian private Radoje Ljutovac fired a cannon at three enemy aircraft and brought one down. The cannon was a slightly modified Turkish cannon captured during the First Balkan War in 1912.
The earliest known weapon made specifically for the anti-aircraft role was the Ballonabwehrkanone, or balloon-defence cannon, during the Franco-Prussian War of 1870. Gustav Krupp mounted a modified 37 mm one-pounder gun on a horse-drawn carriage to shoot down French balloons after Paris was besieged.
Post-war analysis showed that even with the newest systems, about 90% of bombers still reached their targets, a figure made unacceptable by the nuclear bomb. Guided missiles took over from guns, with the US adopting the Nike Ajax and the USSR the SA-2 Guideline, and by 1955 the US deemed the 40 mm Bofors obsolete against jet aircraft.
Flak comes from the German word Flugzeugabwehrkanone, meaning aircraft-defence cannon. It is one of several names for anti-aircraft fire, alongside terms like AA, ack-ack, and archie, the last a First World War British term believed to derive from music-hall comedian George Robey's line, Archibald, certainly not.
Stealth designs cut detection ranges so far that aircraft are often never seen, so anti-stealth technology uses bistatic radars, low-frequency radars, advanced thermographic cameras, and high-sensitivity radars to locate them. The Russian S-400 is claimed to detect a target with a 0.05-square-metre radar cross-section from 90 km away.