A flamethrower is a mechanical incendiary device designed to project a long, controllable stream of fire. They were first used by the Greeks in the 1st century AD. In modern times, they were used during World War I, and more widely in World War II.
Most military flamethrowers use flammable liquid thickened into a substance similar to napalm, but commercial flamethrowers tend to use high-pressure propane and gasoline, which is considered safer, as they both die out faster and are easier to put out. Note that napalm was never used with flamethrowers. In comparison, a liquid flamethrower's fuel sticks to its targets and is harder to put out with water, while also allowing for a more specific burn effect. Napalm specifically also quickly deoxygenates the surrounding air, making smoke inhalation or asphyxiation a real threat.
They are used by the military and by people needing controlled burning capacity, such as in agriculture (e.g., sugar cane plantations) or other such land management tasks. They can be designed to be either carried by the operator or mounted on a vehicle