Gunpowder is a black powder, and the earliest-known chemical explosive. It contains sulphur, charcoal and saltpetre (a chemical called potassium nitrate obtained from salt beds). The sulphur and charcoal are the fuel, while the saltpetre provides the huge amounts of oxygen needed to cause the very rapid chemical reaction that we call an explosion.
When heat is applied to this mixture, using a fuse, it burns rapidly, producing a large amount of gas (and heat). The gas expands quickly and pushes everything out of the way.
What is important to know is that gunpowder is simply a rapidly-burning substance. If it is placed inside a container so that the gases cannot escape, then it will blow up the container and everything nearby. That is why the Gunpowder Plot was based on barrels of gunpowder. Similarly, gunpowder works to shoot bullets because the gunpowder is contained in the gun and the only way the gases can escape is by pushing the bullet out of the way.
Gunpowder was invented by the Chinese who used it almost immediately to make fireworks as well as gunpowder weapons.
The idea of using gunpowder then spread westwards through the Middle East to Europe by the early Middle Ages. However, it was not known at the start of the Middle Ages, which is why castles were built. As soon as gunpowder and cannon became widely available, castles in their original form were useless as the walls could be blown up (using barrels of gunpowder in tunnels under the walls) or blown down by cannonballs. All later castles were forts, with massive sloping earth banks to stop direct hits by cannonballs. Berwick and Carlisle castles (Northern England) show this superbly.
Gunpowder does not send out a shock wave if burned in the open air. Most explosives used nowadays send out shock waves and so are called high explosives.
