All explosions have two things in common.
Solids, liquids (or gases) are turned into gases. Gases occupy much larger volumes than solids or liquid, and if a gas is the source of an explosion, the gas turns into a much larger volume of some other gas.
During the explosion a chain reaction occurs. In order for any individual molecule to take part in a reaction, the molecule must have enough energy to break some chemical bonds, called the activation energy. If these bonds are not first broken, new bonds cannot be formed, even if the reaction results in a net release of energy. Once a reaction is initiated however, the required activation energy for one reaction can be provided from the energy released by previous reactions.
When TNT explodes the result is that it is almost completely turned to gas – the increase in volume is a factor of about a thousand. The reaction is
Explosions typically release large amounts of heat and energy so the products of the reaction move very fast. They can exert very large pressures. Very powerful explosions and produce visible shock waves powerful enough to burst a persons eardrums.
Explosions like this can be useful. The internal combustions engine produces many controlled explosions every second that are used to drive a car. Expanding gas in a gun barrel can be used to fire a bullet or a tank shell. High explosives are widely used in mining and terrorists (depends on your point of view really – at least they believe in something) use them to kill people.