One of the
signature differences between humans and animals is that we use weapons. Soft and weak, people wouldn’t survive unprotected in any environment on the planet without tools. The first tool was a
weapon.
From the stone as hammer came the hand axe, which when mounted on a stick evolved into the spear. Once humans perfected the art of thrusting something sharp on a long stick at one another, they began to think of better ways to make their sharp point reach the enemy first.
Taking flight
After the thrown spear, projectile technology progressed from muscle-driven weapon systems such as the atlatl and bow-and-arrow to muscle-storage devices like the crossbow, onager, and ballista. (The trebuchet is a kind of muscle-storage system, but muscle power is only used to “cock” the weapon; the counterweight provides the energy that throws the projectile.)
The concept of using chemical reactions to hurl projectiles first originated in the gunpowder rocket, whose first recorded use was by the Chinese thousands of years ago. The idea to put the gunpowder in a tube and use that configuration to hurl a projectile came shortly after. (There are also guns that use compressed air or gas to fire a projectile. Tennis-ball butane cannon use a chemical reaction.)
Cannon everywhere
Cannon were originally valued most for their impressive noise, fire, and smoke, which made for a dramatic show that demoralized the enemy more than it actually hurt him physically. As the technology behind both powder and gun metal improved, cannon matured to where they became the terror of the battlefield they are today.
One of the technology spin-offs of advanced cannon development was smaller, lighter cannon. Cannon could be made so small that a single soldier could carry and fire one all by himself.
Once portable cannon became commonplace, the world changed.
PS: The term “gun” is also used for any gun-like device, especially if it shoots something out like labels or laser beams.