Short answers, pulled from the story.
An explosive is a reactive substance that holds a great amount of potential energy that can produce an explosion if released suddenly, usually with light, heat, sound, and pressure. The stored energy may be chemical, like nitroglycerin or grain dust, pressurized gas, like a gas cylinder or aerosol can, or nuclear, as in the fissile isotopes uranium-235 and plutonium-239.
High explosives detonate, meaning the reaction front moves through the material faster than the speed of sound, while low explosives deflagrate, with a reaction front slower than the speed of sound. High explosives detonate at about 3 to 9 km/s, with TNT around 6.9 km/s and C-4 about 8.0 km/s, while low explosives like gunpowder are usually employed as propellants.
Primary explosives are extremely sensitive to impact, friction, heat, or static electricity and are used in tiny amounts in detonators and blasting caps. Secondary explosives such as TNT and RDX need substantially more energy to initiate, and tertiary explosives, also called blasting agents, are so insensitive that they require a secondary explosive booster and are favored by large-scale mining and construction operations.
Gunpowder, the first form of chemical explosive, first saw use in warfare in 1161. Nitroglycerin, the first explosive stronger than black powder to see widespread use, was developed in 1847.
Heat, sunlight, electrical discharge, and water all reduce an explosive's stability. Most explosives become dangerously unstable above 70 degrees Celsius, ultraviolet sunlight rapidly decomposes many nitrogen-bearing compounds, static or spark discharge can cause a reaction, and moisture reduces sensitivity, strength, and velocity of detonation while corroding metal containers.
The Explosives Act of 1917 was the first federal regulation of licensing explosives purchases, signed on the 6th of October 1917. The Organized Crime Control Act of 1970 transferred many explosives regulations to the Bureau of Alcohol, Tobacco and Firearms effective in 1971, and current rules are governed by Title 18 of the United States Code and Title 27 of the Code of Federal Regulations.