Flamethrowers are devices that contrary to belief, throw napalm (not flames). Think of napalm as when gasoline met KY jelly and had a child. The main ingredients that are added to the gasoline to make it jellied are what make up its name. Napthenic and Palmitic acids Gasoline = NAPALM.
Flamethrowers are devices that use high pressure gasses (like nitrogen) to propel napalm from the nozzle of the flamethrower to its target. At the end of the nozzle there is usually a pilot flame (think of a bic lighter or zippo) that is used to ignite the jelly and reduce everything that napalm touches into burn raisins. Flamethrowers are extremely effective at taking fortified positions such as pillboxes and caves. The only problem with this whole concept is that a flamethrower has only a few seconds of fuel to use before it's useless. Even worse is that whomever is using that flamethrower is the target of every enemy on the battlefield due to the whole "Hey guys, I'm shooting a 40 foot long stream of bright liquid death" thing.