Laymen are prone to calling anything that comes apart faster than perception an explosion, or alternately they say, "it exploded". We have experience in everything a layman might term an explosion. True explosions fall into three categories: detonations, deflagrations and BLEVEs.
Detonations are caused by high-order explosives and fuel-air mixtures under certain circumstances. Detonations travel at the speed of sound in the material that is detonating. This means that the energy that is spreading out from the initial stages of the detonation cannot get out ahead of the advancing detonation reaction zone. As a result, all of the energy goes into one sharp wave. Detonations can sever steel, and send fractures into hard rock. Detonation waves are so sharp that they may cause spalling on the leeward side of walls, for the same reason that, after impact, the far ball swings away in a Newton's Cradle . We have experience with detonations involving commercial and military explosives and also fuel-air mixtures. True detonations in losses are relatively rare. The tell-tale signs of a detonation are fragmentation of debris, in short: smithereens.
Deflagrations are caused by low order explosives, confined propellants (rifle powders) and most fuel-air mixtures. Flame speeds are less than the speed of sound in the material that is deflagrating. Consequently, the energy from the early phase can get out in front of the flame front. The flame front then builds onto the back of the pressure wave, creating a longer-lasting smoother pressure wave. Such a pressure wave may overload structures, and cause separations and failures. Deflagrations are also more effective at heaving large pieces of debris. The tell-tale sign of a deflagration is the dis-assembly of debris into large sections. We have experience in cases of deflagration "explosions".
Anyone who has experienced a detonation first hand would not confuse this type of explosion with any other. There is a visible wrinkle in the air that races out from the seat of the detonation -- this is the shock wave. Before the shock wave hits, there is silence. When it hits, even at a distance, it feels like the air turns to concrete for an instant and something solid has hit. If a deflagration feels like the impact of a breaking ocean wave, or running into a snowbank at highway speed, a detonation wave feels more like a tennis racket flat to the chest. But that is just what it feels like from a safe distance. To quote an insured: "You know how people say a tornado sounds like a freight train? Those people were not all that close." As part of safety training on the Defence Research Establishment Suffield Experimental Proving Ground, experimenters are taught that a detonation wave of just 4 pounds per square inch overpressure, a little less than 1/3rd of an atmosphere, was generally sufficient to be fatal by internal injuries from the sharp atmospheric concussion alone.
A boiling-liquid expanding-vapour explosion (BLEVE) may or may not involve any combustion depending on whether the vapour is also combustible. Superheated water in pressure vessels can BLEVE instantly turning to an expanding steam cloud causing extensive damage. We have experience with BLEVE "explosions" that were followed up by combustion of the vapour cloud; Superior Propane is the most prominent case of this type in our experience. Even though BLEVEs do not necessarily release any chemical energy like detonations and deflagrations do, BLEVEs of superheated water, for instance, can be extraordinarily devastating owing to the mass of material involved. Momentum and imparted forces are proportional to mass. It is not uncommon for an industrial tank to hold many tons of hot water or other material that might support a BLEVE. In contrast, the total mass of air and methane in a house that suffers a gas explosion or deflagration might be only a dozen kilograms of methane and a fraction of a ton of air.
Laymen may also report a rupture as an explosion. A rupture is the release of mechanically stored energy. For instance, an over-inflated or over-loaded tire can rupture. Since the rupture-failure of a tire seems very sudden, noisy and damaging to the tire, a layman would often say the tire exploded.
Vessels and tanks can rupture if the pressure inside exceeds the strength of the tank. Ruptures of tanks where the contained material is liquid in the tank and would tend to be gaseous in free air can either be BLEVEs or may result in the release of super-cold (cryogenic) liquid. For instance, if a propane tank ruptures in a fire, it is probably a powerful BLEVE followed up by a powerful vapour deflagration. If a propane tank is over-pressurized at more ordinary temperatures, the tank will rupture and a fraction of the propane might boil and expand in a cloud in a partial BLEVE; however, the majority of the mass of the propane will gush out as a cold liquid at about -42C. The rupture of tanks where the material inside does not change state are generally called ruptures, though the rupture of tanks of gases at very high pressures can be very damaging.
Solids can rupture. A concrete column under very high compression will suddenly spall to the sides throwing off chunks of concrete. In one case where a machine was running out of control, turning at very high speed, the forces in the metal from the spinning exceeded the strength of the material and the machine's chuck ruptured -- most laymen would say "exploded".
Any rapid conversion or release of energy may lead to a rapid scattering of debris. A layman might refer to this as an explosion though it is not. For instance, when a lot of heavy material falls, the release of a lot of gravitational energy can cause the debris to have a wider range effect when it hits the ground. Certainly if a crane were to drop a skid of bricks, the pile of bricks will seem to explode to the sides when the skid hits the ground. Such conversions of energy can be very energetic and damaging. For example, the uncontrolled collapse of the World Trade Center towers caused a rapidly-expanding cloud of energetic debris that affected all of the buildings around. It really did look like a bomb had hit the buildings around. The airborne debris that blasted in through the windows and accumulated feet-deep in places, even 8 storeys from the ground, looked like volcanic debris.