Short answers, pulled from the story.
A weapon, arm, or armament is any implement or device used to deter, threaten, inflict physical damage, harm, or kill. In a broader sense, a weapon can be anything used to gain a tactical, strategic, material, or mental advantage over an adversary.
The earliest unambiguous weapons are the Schöningen spears, eight wooden throwing spears dating back more than 300,000 years. Possible evidence of weapon use also appears at Nataruk in Turkana, Kenya, where skeletons from 10,000 years ago carry obsidian projectiles embedded in the bones, though that interpretation has been challenged.
Gunpowder, introduced from Asia, revolutionized warfare by replacing the trebuchet with the cannon as the dominant siege weapon. The Ottomans used cannon to destroy much of the fortifications at Constantinople, and firearms release energy from combustible propellants rather than from a counterweight or spring.
Poison gas was banned because it did not win battles, generals did not want it, and soldiers hated it more intensely than bullets or explosive shells. The Geneva Protocol, prohibiting asphyxiating, poisonous, and bacteriological methods of warfare, was issued in 1925 and accepted as policy by all major countries.
Mutual assured destruction, known as MAD, is the realization that all-out nuclear war is no longer a survivable scenario. After the United States and the Soviet Union built enough nuclear capability to destroy the Earth a hundredfold, armament funding shifted back toward conventional arms for limited wars.
Weapons are classified by user, by function, and by target. Classification by user includes personal, light, heavy, and crew-served weapons, while classification by function covers categories such as firearms, biological, chemical, edged, explosive, and nuclear weapons, and classification by target includes anti-aircraft, anti-tank, and anti-ship weapons.
The arms industry is a global industry covering research, development, engineering, production, and servicing of military material. According to the research institute SIPRI, the arms sales of the world's 100 largest private arms-producing and military services companies totaled $420 billion in 2018.