An explosion is different from a fire. In an explosion, a chemical reaction takes place which causes the materials making the explosion to expand very violently. What is happening is that a combination of solids and liquids creates a gas under great pressure.

The gas then expands at a speed faster than the speed of sound. As a result, the gases break the sound barrier, and we hear this as a loud bang.

In many case we also feel the shock waves of an explosion. Indeed, the explosion is designed to make nearby objects break up. Explosions are use to shake buildings down.

Explosions do not have anything directly to do with fire. Fires may occur after an explosion, but that would be due to the fact that something has got very hot. Indeed, in places such as an oil well fire, explosions are use at the top of the wellhead to blow out the fire. They simply blow the burning gases off the wellhead, giving time for the unburned gases to be capped off before they catch fire again.

Explosions happen inside bullets and rifle shells, in the material packed into the barrel of a cannonball, with dynamite, inside fireworks, and many other places.

One of the reasons that explosions are often followed by fires is that gas pipes are broken by the explosion, and the gas catches fire.