Gunpowder is a substance which burns very rapidly and is used as a propellant in firearms. There are two types:
Both forms of gunpowder burn, producing a subsonic deflagration wave rather than the supersonic detonation wave which high explosives would produce. This reduces peak pressures in a gun, but makes it less suitable for shattering rock or fortifications. However, black powder was, for a couple centuries, the only blasting agent available.
Although historically gunpowder has meant black powder, as in the Gunpowder plot, modern references usually imply smokeless powder when referring to the propellant in small arms.
Gunpowder is also a sort of green tea.