Detonation (Lat: detonare, de + tonare, "to expend thunder") is caused by an extremely fast chemical reaction that is accompanied by the propagation of a shock wave, an extremely intense variation of pressure (which may reach several tens kilobars). The very existence and the propagation of this shock wave beyond the explosive, into its surroundings, is what confers to the detonation its violently destructive effects. Detonation is caused by the nearly instantaneous character of the reaction; once the shock wave is triggered, the reaction stops only after nearly total exhaustion of the material. Outside the explosive, the shock wave propagates like the wave caused by an aircraft passing the sound barrier. Its intensity decays progressively since it loses is energy. However, during its active trajectory, it causes important or catastrophic damages, depending on the nature and amount of explosive involved.
The passage of the shock wave caused by a detonation is accompanied by a strong noise. It is speculated that, since two shock waves propagate at the same time, one in the air and the other in the ground, at two markedly different speeds, witnesses of a detonation may hear two noises, both felt well after the light flash has been seen. The time between these events should be proportional to the distance of the observer from the explosion.
Deflagration (Lat: deflagrare, de + flagrare, "to burn down") is a technical term describing subsonic combustion that usually propagates through thermal conductivity.
A deflagration is the set of phenomena accompanying the rapid passage of a reaction front, e.g., the front of a flame (combustion of a gas or a vapor, more rarely of a solid). In a homogeneous mixture of air and a combustible gas or vapor, a flame propagates at a constant velocity that is high but remains of the same order of magnitude of many familiar phenomena. It is of the order of 1 to 10 feet/second, hence comparable to that of a walker or a runner (as opposed to detonation that propagates several times faster than sound in air). A deflagration takes place during the rapid inflammation of the mixture of air and gas above a burner in a stove. If the amount of gas is small, this is uneventful; if the amount is important, the result may be an explosion. The flame of a gas burner is a deflagration moving at a constant velocity, in the direction opposed to that of the gas flux. In a deflagration, the combustion products move at a subsonic velocity, in the direction opposed to that of the flame. In numerous cases, the propagation of a flame in an explosive gas mixture having a proper composition may accelerate suddenly.