A flamethrower is a mechanical device designed to project a long controllable stream of fire.
Some flamethrowers project a stream of ignited flammable liquid; some project a long gas flame. Most military flamethrowers use liquids, but commercial flamethrowers tend to use high-pressure propane and natural gas, which is considered safer. 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.

Military flamethrowers

History
The concept of throwing fire has existed since ancient times.
Greek fire, extensively used by the Byzantine Empire, is said to have been invented by a Syrian Jewish refugee named Kallinikos (Callinicus) of Heliopolis (Syria), probably about 673. The flamethrower found its origins also in the Byzantine Empire, employing Greek fire in a device of a hand-held pump that shot bursts of Greek fire via a siphon-hose and piston, igniting it on a match on its way out, in a manner like its modern versions.

Early history
The English word 'flamethrower' is a loan-translation of the German word Flammenwerfer, since the modern flamethrower was first invented in Germany. The first flamethrower, in the modern sense, usually is credited to Richard Fiedler. He submitted evaluation models of his Flammenwerfer to the German army in 1901. The most significant model submitted was a man-portable device, consisting of a vertical single cylinder 4 feet (1.2 m) long, horizontally divided in two, with pressurized gas in the lower section and inflammable oil in the upper section. On depressing a lever the propellant gas forced the inflammable oil into and through a rubber tube and over a simple igniting wick device in a steel nozzle. The weapon projected a jet of fire and enormous clouds of smoke some 20 yards (18 m). It was a single-shot weapon - for burst firing, a new igniter section was attached each time.

20th century
It was not until 1911 that the German army accepted the device, creating a specialist regiment of twelve companies equipped with Flammenwerferapparate. Despite this, the weapon went unused in WWI, until 25 June 1915 when it was briefly used against the French. On 30 July 1915 it was used against British trenches at Hooge, with limited, but impressive, success.
The weapon had drawbacks: it was cumbersome and difficult to operate and could only be safely fired from a trench, so limiting its safe use to areas where the opposing army trenches were less than 20 yards apart, not a common situation. Nevertheless, the German army continued deploying flamethrowers during the war in more than 300 battles, usually in teams of six flamethrowers.
In WWI it was usual to shoot burning enemy soldiers, to save them the agonizing death by incineration; but in WWII such mercy was uncommon, unless they were a danger to one's own troops.

WWI
The flamethrower was extensively used during World War II. In 1940, the Wehrmacht first deployed man-portable flamethrowers to destroy Dutch gun emplacements and fortifications. Subsequently, in 1942, the U.S. Army introduced its own man-pack flamethrower.
The vulnerability of infantry carrying backpack flamethrowers and the weapon's short range led to experiments with tank-mounted flamethrowers (flame tanks). The British hardly used their man-portable systems, relying on special Sherman, Churchill, and Matilda tanks in the European theatre. These tanks proved very effective against German defensive positions, and caused official Axis protests against their use. There are documented instances of German SS units executing, out-of-hand, any captured British flame tank crews.

WWII
See http://www.diggerhistory.info/pages-weapons/flamethrowers.htm : history and images

British
The Germans made considerable use of the weapon (Flammenwerfer 35) during their invasion of western Europe, especially in Holland and France, against fixed fortifications, but it soon fell into disfavor, except in reprisal operations. Yet, on the Eastern Front its battlefield and "scorched earth" tactic uses continued until the end of the war. See the Stroop Report link on article of the 1943 Warsaw Ghetto Uprising.
WWII German army flamethrowers tended to have one large fuel tank with the pressurizer tank fastened to its back or side. Some WWII German army flamethrowers occupied only the lower part of its wearer's back, leaving the upper part of his back free for an ordinary packful of supplies.
As the Third Reich was deteriorating at the end half of WW2, a smaller compact flamethrower known as the Einstossflammenwerfer 46 was produced due to the lack of materials and funds.
External link with images: http://www.lonesentry.com/articles/portft/

German
In the Pacific theatre, the US Marines used the backpack-type M2A1-7 flamethrower and M2-2 flamethrowers, finding them especially useful in clearing Japanese trench and bunker complexes. In cases where the Japanese were protected from the flames by deep caves, the burning flames often consumed the available oxygen, suffocating the occupants. The Marines eventually stopped using their infantry-portable systems with the arrival of adapted Sherman tanks with the Ronson system (c.f. flame tank). The U.S. Army used flamethrowers in Europe to clear out German bunkers, notably during Operation Overlord.

USA
Some Soviet Army flamethrowers had three backpack fuel tanks side by side. Some descriptions seem to say that its user could fire three shots, each emptying one of the tanks.
Unlike the flamethrowers of the other powers during WWII, the Soviets were the only ones to consciously attempt to camouflage their flamethrowers, The ROKS-2 flamethrower which was done by disguising the "gun" as a standard issue rifle, such as the Mosin Nagant, and the fuel tanks as a standard infantryman's rucksack, to try to stop snipers from specifically targeting flamethrower operators.

USSR
The United States Marines used flamethrowers in the Korean War and the Vietnam War.
Flamethrowers have not been in the U.S. arsenal since 1978, when the Department of Defense unilaterally stopped using them, because of public opinion concerns that found their use inhumane, although they are not banned in any international treaty the U.S. has signed. Thus, the US decision to remove flamethrowers from its arsenal is entirely voluntary.

After 1945
In the United States, private ownership of a flamethrower is not restricted by federal law, but is restricted in some of its states, such as California, by state laws (c.f. California Health and Welfare Codes 12750-12761, Flamethrowing Devices) CA H&W Code on line
In California, unlicensed possession of a flame-throwing device —statutorily defined as "any nonstationary and transportable device designed or intended to emit or propel a burning stream of combustible or flammable liquid a distance of at least 10 feet" H&W 12750 (a)— is a misdemeanor punishable with a county jail term not exceeding one year OR with a fine not exceeding $10,000 (CA H&W 12761). Licenses to use flamethrowers are issued by the State Fire Marshal, and he or she may use any criteria for issuing or not issuing that license that he or she deems fit, but must publish those criteria in the California Code of Regulations, Title 11, Section 970 et seq. CA Regs (CA H&W 12756) (definitions and scope, administration, enforcement and penalties)

Private ownership
Flamethrowers also are used by people needing controlled burns, as in agriculture and other land management tasks. In ripe canebrakes of sugar cane, they are used to burn up the dry dead leaves which clog harvesters, and incidentally also kill any lurking poisonous snakes. Flamethrowers are also sometimes used for igniting controlled burns of grassland or forest, although more commonly a driptorch or a flare (fusee) is used.
U.S. troops used flamethrowers on the streets of Washington D.C. to clear snow (as mentioned in a December 1998 article in San Francisco Flier), one of several clearance methods used for the surprisingly large amount of snow that fell before the presidential inauguration of John F. Kennedy. A history article on the U.S. Army Corps of Engineers notes, "In the end, the task force employed hundreds of dump trucks, front-end loaders, sanders, plows, rotaries, and flamethrowers to clear the way".[1] The massive effort by city, military, and others even included 1700 Boy Scouts. The work paid off the next day, January 20, 1961, with JFK's successful inauguration.
Flamethrowers are also used for special effects, such as concerts and special events; particularly, the band Rammstein's lead singer Till Lindemann is known to use a flamethrower during live performances.

Other uses
Due to the flamethrower's spectacular effect, it is often featured in fiction, action movies, and, in video games, even where in reality it would not be used.
Hollywood seems to have no difficulty getting hold of flamethrowers, but, on-set, for the safety of the actors, they often are filled with propane gas, instead of liquid fuel, producing a visually similar (though rarely identical) flame effect, but without the spray of fuel, splatter of flame, dense smoke, and area effect of the genuine fuel, i.e. in the Omaha Beach sequence of Saving Private Ryan, the exploding flamethrower was filled with enough propane gas to burst the containers and produce spectacular flames. The explosion seen occurred seconds after the tank burst: it was caused by blowing vaporized propane onto the explosion (the cloud is visible in the finished film as a billowing white cloud in front of the actor).
The same basic effect was created by Stan Blackwell and his crew during the FOX drama 24's fifth season.
Behind the scenes in filmmaking flamethrowers are used as special effects tools in simulating fires, explosions, volcanic eruptions, et cetera.

In the first Alien film, it is Ellen Ripley's primary weapon against the Xenomorphs.
In Lethal Weapon 4 a criminal uses a flamethrower.
In the game General Mayhem flamethrowers are used by some soldiers.
In John Carpenter's The Thing, the flamethrower is often used against the shape-shifting alien creature.
In the movies The Exterminator and Exterminator 2, Johnny Eastland (Robert Ginty) uses a flamethrower for wiping out criminals.
In Metal Gear Solid 3: Snake Eater the flamethrower is used by The Fury, a member of the Cobra Unit during the battle against Naked Snake.
In the game Company of Heroes, both the Wermacht and US sides feature infantry- and vehicle-portable flamethrowers.
In the game Red Faction the player can weild a compact flamethrower and uses the fuel canister as a firebomb.
In the game Starcraft, the Terran "Firebat" unit is a ground unit with dual-wielded flamethrowers on either arms.
In the Command & Conquer: Tiberian series, the Brotherhood of Nod has used flame weapons since the First Tiberium War.
In Wolfenstein: Enemy Territory the flamethrower is a popular weapon. A lot of players disapprove of its use and often call players who use it flamers.
In several games of the Grand Theft Auto series (Grand Theft Auto II, Grand Theft Auto III, Vice City, and San Andreas), the flamethrower is an available weapon.
In Killzone: Liberation the flamethrower is available for selection after finding a certain amount of collectables.
In the game Aliens vs. Predator 2 the flamethrower is a weapon very effective against the xenomorphs.
In the PC version of Halo: Combat Evolved the flamethrower is available only in multiplayer mode. However, in Halo 3 the flamethrower will be featured in both campaign mode and multiplayer.
In the Xbox version of Conker: Live & Reloaded, in multiplayer mode there is a class called "thermophile" which wields a flamethrower.
In Resistance: Fall of Man the flamethrower is a weapon available in multiplayer and campaign mode on the second playthrough. It is highly effective against the Chimera.
In Garth Marenghi's Darkplace, Dr. Rick Dagless, MD sets the screaming Larry Renwick on fire with a flamethrower, after a barrage of shots from himself and Thornton Reed.
Garfield uses a flamethrower to clean out his refrigerator.
Repairman Jack uses a flamethrower to defeat the evil Rakoshi at the end of F. Paul Wilson's novel The Tomb
In the anime Pumpkin Scissors the flamethrower is a weapon, banned by international treaties, but secretly used by Empire
in Last War, mainly by 908 HTT platoon (High Temperature Troopers).
in Halo 3 flamethrowers may be wielded against the Covenant (Halo)
in Halo PC flamethrowers are usable in multiplayer matches.
in the Timesplitters series, the flamethrower is a prominent weapon.
In Resident Evil (video game), the character Chris Redfield can momentarily pick up a flamethrower in a mining area to unlock certain passages and wield it against monsters only for a limited amount of time. In fiction

Converted Flamethrower 40 (fires a 0.5% solution of CN gas in water as a riot control weapon) See also