Short answers, pulled from the story.
Gunpowder is a mixture of sulfur, charcoal, and potassium nitrate, also known as saltpeter. The current standard recipe adopted in 1780 is 75% potassium nitrate, 15% softwood charcoal, and 10% sulfur. The sulfur and charcoal act as fuels, while the saltpeter is the oxidizer.
Gunpowder was invented in China, where it is counted among the Four Great Inventions. The first confirmed reference appears in the 9th century during the Tang dynasty, in a formula contained in the Taishang Shengzu Jindan Mijue in 808. It was originally developed by Taoists for medicinal purposes.
Gunpowder is classified as a low explosive because of its relatively slow decomposition rate, low ignition temperature, and low brisance. It deflagrates, burning at subsonic speeds, rather than detonating like a high explosive, which produces a supersonic shockwave. This makes it a good propellant but poor at shattering rock or fortifications.
Gunpowder's first recorded military application dates to 904, in the form of incendiary projectiles. By 1083 the Song court was producing hundreds of thousands of fire arrows, and fire lances were first recorded at the Siege of De'an in 1132. By 1287 at the latest, the technology had become the true hand cannon.
The Mongols introduced gunpowder during their invasions of Persia, Mesopotamia, and India. The Muslims acquired knowledge of it between 1240 and 1280, and the Syrian writer Hasan al-Rammah called saltpeter Chinese snow and rockets Chinese arrows. The earliest Western accounts were written by Roger Bacon in 1267.
Besides firearms and artillery, gunpowder was used as a blasting powder in quarrying, mining, and road construction, and in fireworks, signal flares, and rockets. It was originally produced as a medicine, eaten for digestive ailments and rubbed onto rashes. British sailors even used it for tattooing when ink ran out.