Flamer
The Flamer is a short-ranged support weapon employed in many different variations by nearly all of the races of the WH40K universe.
Overview
The flamer is a weapon that sprays an area with extremely flammable liquid/gas, burning anything in front of it. Now apart from the varying degrees of burns you'll receive when you're in the business end of one, your squad won't really be motivated when they see a giant firewave of painful and slow death and see that the rest of them are experiencing said pain and death so they'll probably run for it... unless you're fighting with anyone short of adamantium-forged testicles where they'd probably charge the gunner for vengeance.
Armies field flamers in different sizes ranging from pistol-sized flame units, a regular rifle sized unit to a fuckhueg flamer capable of bathing entire platoons in fire with a single burst. Table-top wise, Flamers are special weapons that ignores cover bonuses and extremely effective against infantry. Very good against the Tyranids due to their total lack of vehicles, so you always have a chance at hurting most any tyranid unit you shoot at, plus it's great for killing all those pesky rippers.
Imperial Variations
A cruel and painful weapon in its own right, this would not be 40k if there were no several variations of the flamer, each deadlier than the last.
Flamer Pistol
Mainly used by the Adepta Sororitas and the Inquisition, this weapon is a pint-sized flamethrower that can be wielded as if it were a pistol. Some extremely unhinged individuals go as far to dual-wield them for extra burny power.
Flamer
The regular sized flamethrower as described above, used by nearly everyone in the 41st millenium.
Combi-Flamer
The Orks have proven that any weapon can be made more dakka by slapping another weapon onto it. Combining this with the Imperium's love for flame weapons, the combi-flamer is a weapon that can be strapped onto a regular Bolter to allow for a single gout of fire when needed.
Heavy Flamer
The big brother of the regular flamer, able to do far more damage. Mainly used by Terminators and vehicles, they are also carried by the Sisters as a heavy weapon, and by the Catachans because they simply don't give a fuck.
Flamestorm Cannon
An even bigger variation of the flamer, found on the Land Raider Redeemer as a sponson weapon and the primary weapon of the Baal Predator.
Inferno Cannon
The primary weapon of the Hellhound, this weapon focuses on the "thrower" aspect of "flamethrower". Similar in strength to the Heavy Flamer, the Inferno Cannon can project its payload over a considerable distance.
Inferno Gun
Some men just want to watch the world burn. Others provide the matches. The Inferno Gun falls under the second category. Mounted only on the Warhound Scout Titan and the Malcador tank it will do horrifying damage against anything caught in its path.
Alien Variations
Thought the Imperium has by far the most flame weapons, certain aliens also employ them.
Eldar Flamer
Used by Storm Guardians to support their role as close-quarters combat and by Wraithlords as secondary weapons for assault-oriented constructs.
Dragon Breath Flamer
Used by exarchs of the Fire Dragons, these weapons can sear anything that gets within range of their wielder.
Ork Flamer
The Orks employ Burnas for their flamer-duties. These weapons are unique because they can also be used as cutting torches to deal serious damage in close combat.
Tau Flamer
Used soley by Crisis Battlesuits, they are meant to deter close-range attackers. Due to the nature of said battlesuits, they are not intended to be used as a primary weapon.