The most common artificial explosions involve chemical explosives. These are caused by anything that once ignited, burns extremely rapidly and produces a large amount of hot gas in the process. This hot gas expands very rapidly and creates pressure. Explosives are classified by the amount of energy it takes to initiate the explosive reaction. This initial energy can be anything from a shock, an impact, a friction, an electrical discharge or the detonation of another explosive. Chemical explosives are also divided into high explosives and low explosives, though some explosive materials can fall into either category according to how they are initiated.
Low Explosives burn through deflagration rather than a detonation wave and are usually initiated by heat and require confinement to create an explosion. High Explosives have a supersonic reaction and will explode without confinement, they are compounds initiated by shock or heat with Brisance. With a Brisant explosive the maximum pressure is attained so rapidly that a shockwave is formed and the result will destroy the material surrounding it or in contact with it.
Brisance is important when determining the effectiveness of an explosion, like fragmenting shells, bomb casings, grenades etc.
Primary Explosives are extremely sensitive and require a small quantity of energy to be initiated. They are mainly used in detonators to initiate secondary explosives. Secondary Explosives are relatively insensitive and need a great amount of energy to initiate. They have much more power than primary explosives & require detonators to work, examples are: Dynamite, TNT, RDX, PETN, HMX, ammonium nitrate, tetryl, picric acid, nitrocellulose, gelignite.
All of these triggers are important to think of when you start to animate an explosion, the reasons for it starting and the materials involved will have a big impact to the look and feel of the explosion. Remember that explosions don't occur in isolation, secondary explosions and impacts can be created from the initial explosion and will happen when other material is ignited within the surrounding blast area.