Explosives:

An explosive is any solid, liquid, or gas that when subjected to stimuli such as shock or heat undergoes a rapid chemical reaction and converts into gaseous form. This chemical reaction releases heat and pressure equally in all directions. Explosives fall into three categories, low explosives, high explosives and primary explosives according to the detonating velocity or speed at which the chemical reaction takes place as well as their role in the reaction. I recommend that the operative read the US Army FM 5-250, which is an excellent resource of information on the handling and application of military and commercial explosives.
Low Explosives
Low explosives change from a solid to a gaseous state relatively slowly over a sustained period of time (up to 1,300 feet per second). These are generally the easiest explosives to manufacture. The effect that occurs in low explosives when they are ignited is called "deflagration" which is actually a very rapid burning rather than the "detonation" which occurs in high explosives. Low explosives are usually ignited by a spark or flame. Because of the their slow burning explosion they must be confined in a strong container in order to produce a powerful blast. If ignited without proper containment they produce flame and smoke but no explosion.
Low explosives are really only effective against soft targets such as light buildings, unarmored vehicles and, of course, personnel. Examples of low explosives are Black powder, Smokeless powder, and Chlorate powder.

High Explosives
Unlike low explosives, high explosives undergo a "detonation" when initiated. This detonation is an almost instantaneous chemical reaction at a speed of between 3,000 an 28,000 feet per second depending on the type of explosive. A blasting cap or detonator is required to initiate this reaction. A "low order" detonation may occur when an insufficiently powerful detonator is used or when the explosive mass is loose or degraded. Characteristics of a "low order" detonation include relatively little blast, lots of smoke and little target damage.
High explosives are essential for effective attacks on buildings, armored vehicles, bridges and most other modern infrastructure. Examples of high explosives are TNT, Dynamite and C-4.

Primary Explosives:
Primary explosives are highly sensitive compounds which are used in detonators and small arms primers. The are easily detonated by heat, spark, impact and friction. Because of this sensitivity they are only used in small amounts, usually less than a gram, in detonators. This tiny amount of explosive is used to initiate a larger charge of less-sensitive explosive, the "base" or booster charge, in a blasting cap. This base charge, in turn, has sufficient energy output to detonate the main explosive charge or , in some cases, a secondary booster, if an extremely insensitive explosive (such as ANFO) is used.
Examples of primary explosives are HMTD, lead azide and mercury fulminate.