A gun (also called a firearm) is a weapon used to shoot one or more metal projectiles called bullets. Guns made of wood or bamboo are thought to have been made in China around 1000 AD. In the next few centuries they spread to other parts of Asia and to Europe where they were made of metal. Guns largely replaced catapults and bows and arrows when they became strong enough to reach out over longer distances to reliably hit a target. They are of many different sizes. Some are small enough to hold in one hand. Others are large enough to sink a warship.
In military usage, the word "firearm" is used for the type of weapons that soldiers carry. Most of them are either pistols or rifles. These small arms can be carried and used by one person. They have the same basic parts: a trigger, a barrel, and a handgrip or stock. The shooter's finger pulls the trigger to fire the weapon which releases a bullet. The barrel guides the bullet which then flies to the target. The shooter's hand holds the handgrip.
Guns can be very effective weapons. Their bullets fly faster than an arrow or spear. Their high kinetic energy makes them more lethal. They also shoot from far distances. Bullets are usually non-explosive pieces of metal.