Melta
Melta weapons are a type of anti-armor weapon used widely by the forces of the Imperium and Chaos; the Tau, Eldar and Necrons also employ their own variants. Think of infrared ovens IN SPACE, infused with extra RAGE, and shot out using a tremendous amount of infrared light to super-agitate the target, so that it heats up to the point where it's hot enough to melt anything from power-armor to super-heavy tank armor.
The problem with melta-based weapons is that due to the super-high energy consumption and heat generation, it's only usable at short range. In comparison to the equally useful Plasma-based guns, meltas are a lot stronger but have a much shorter range, but then again meltas don't explode in your face on a bad roll, but on the gripping hand, Plasma weapons allow your troops some breathing room since they can be used at medium range so your troops aren't always at the risk of being caught in a vehicle's explosion, murdered in CQC, or ambushed by a nearby gunline after they fire off their guns in one turn.
Though suited towards tank hunting, the melta-based guns are good weapons for all-around attack, defense and infiltration, though it falls short in terms of range. Infiltration with this can take vehicles out of the fight before they even have a chance to fire their weapon... maybe that's how the Baneblades were lost?
Depictions of melta weapons firing have been spotty in the face of publications and vidya gaems. In several publications and Dawn of War: Soulstorm, melta weapons fire a constant stream, like a laser-flame thrower. But in the Space Marine video game they fire a single rapid wave of energy (they're also essentially shot-guns; Titus dispersed the beam so he could kill more Orks per shot), and the Dawn of War II series have melta weapons firing a single blue/purple beam, while Dark Crusade uses discrete blobs of melt.
Imperial Melta Weapons
Gamma Pistol
A strange little weapon. You would be fooled to think that it is some sort of Radium weapon. However its function and the ammunition it uses bears more in line with more conventional Melta weapons. Gamma Pistols are a type of weapon used by the Adeptus Mechanicus. The Gamma Pistol is entrusted only to the truly blessed and most honored among the Cult Mechanicus.
A true maser in its own right. The beam of ionizing radiation that leaps from its muzzle can reduce a man to a blackened shadow in a second, but this is a waste of its true strength. Those able to tame its savage machine spirit can cut holes in an even enemy fortifications if necessary.
Inferno Pistol
The Inferno Pistol is a rare pistol-sized melta weapon. Only the Ordo Hereticus, Blood Angels and Sisters of Battle get to use them. Their range is so short that you would have more opportunities to perform a root canal on your enemy in the middle of battle than use them (with even less range than the famous Bruce Lee one-inch punch). Still, those big-hatted pastors love nothing more than the look on your face when they one-shot your Leman Russ. Blood Angels and their successors have a variant called the Infernus Pistol. Sanguinius used one in combination with his Power Sword for close combat, and Dante currently wields an Artificer Infernus Pistol known as the Perdition Pistol.
With their latest codex the Inferno Pistol is now at home with the Blood Angels and available to a variety of their assault specialized units.
Meltagun
The meltagun is the standard armor-breaching "special weapon" available to Imperial armies -- Tactical Squads of Space Marines can take one, Special Weapon Squads of Imperial Guardsmen can take three, basic Sisters of Battle can take two and Sororitas Dominions can take up to four.
Given their fairly quiet and very effective nature, infiltrating parties make use of meltaguns to destroy enemy vehicles before they get a chance to fight on the battlefield. They are also given names such as 'fusion guns', 'melters' and 'cookers'.
Multi-Melta
The multi-melta is essentially two meltaguns strapped together and supercharged, and in game terms only doubles the (already sucky) range and makes it a heavy weapon. Due to their size, multi-meltas are usually mounted on vehicles. The Leman Russ Demolisher may have sponson mounted multi-meltas and the Land Raider Crusader has a hull mounted multi-melta, the weapon fitting with both vehicles' short-range firepower. The Immolator may have twin-linked multi-meltas. They are also found mounted on Land Speeders and their variants, Attack Bikes, Razorbacks, and Dreadnoughts.
This thing is still beastly and tears apart single targets, vehicles or otherwise, but are so huge that only Space Marines and the Sisters of Battle can lift and move them by hand. It should be noted that Sisters of Battle love Melta weapons and, if given the chance, would only ever carry these things and Flamers.
Melta Bomb
For millennia, the Adeptus Mechanicus hypothesized that there was a melta weapon with even less range than the meltagun, and sure enough, one day they found the melta bomb STC printout. Melta bombs are land-mine-sized fusion charges used to destroy enemy vehicles and fortifications; because they have to be placed next to the target to be effective, they are mostly given to assault units who will get up close and personal with the enemy on a regular basis.
Melta Cannon
The Imperial Guard felt left out, and decided to strap two Multi-Meltas together and show up the Sisters by making the Melta Cannon, an EKS BAWKS HUEG Melta weapon strapped to the Devil Dog variant of the Hellhound tank. This thing dumps small blasts worth of Multi-Melta statline upon anything it deems unlucky today, and is one of the best ways of dealing with Terminators. Titans get an even bigger Melta Cannon with absolutely rapetastic statline - it's S10 AP1 10" (yes, TEN inches) blast that always roll extra D6 on partials and 2D6 on direct hits, and due to being primary weapon it also allow you to reroll primary to-pen dice. The only two downsides are its mediocre (for a titan) range and lack of ignores cover, which is silly, because fluff-wise this thing would melt through any wall and bunker like it was made of butter, not to mention some silly ruins and trees.
Magna-Melta
The magna-melta is the Caestus Assault Ram's main ranged weapon. It is primarily intended for cutting through spaceship hulls to deposit its boarding crew inside, though it turns out to vaporize soldiers just fine on the ground.
Melta-Cutter
The Death Korps of Krieg's Hades Breaching Drill uses a melta-drill to dig through the ground. This melta-cutter is also extremely effective at liquifying units above ground.
Siege Melta Array
The Mastodon Superheavy Transport, in addition to its other wonderful bullshit, sports a front-mounted battery of melta weapons, ideal for punching massive holes in the heaviest targets. While this is perfect for opening breaches in fortresses for transport to vomit its 40-man capacity into, it's also an excellent answer to enemy superheavies or just deleting fortifications, as its Stone Burner rule scores extra hits with every Penetrating Hit against a fortification.
Eldar Melta Weapons
The Eldar employ fusion guns, fusion pistols, and fire pikes for their portable anti-tank weaponry. Fusion guns and fusion pistols are meltaguns and inferno pistols in a shape more pleasing to Eldar aesthetics, but fire pikes are longer range (18") meltaguns with no additional downsides. Fire Dragons can take a fusion gun on every squad member and their sergeant can take a fire pike.
Fusion Pistol
A Fusion Pistol is a hand-held, heat-based Melta Weapon utilized exclusively by the Eldar. Essentially the baby brother of the Fusion family.
Fusion Pistols cause the molecules of the target to hyper-vibrate, generating so much heat that their targets burst into flames before suddenly liquefying, and then evaporating into nothingness. Though incredibly short-ranged, the sheer destructive potential of these weapons ensures that they see common usage amongst the masques of the enigmatic Eldar known as Harlequins
Skyraider's Kiss
The Skyraider's Kiss is utilized by both Fire Dragons Aspect Warriors, as well as Storm Guardian units, and is a variant Fusion Pistol that was initially developed for deployment on Bethalmae. The "Skyraider's Kiss" design were "borrowed" from Craftworld Mymeara after their weapons' designs proved capable at functioning at well below-subzero conditions.
The addition of extra cabling to the weapon to shunts extra coolant also allows this variant to be fired more frequently, and requiring less time to cool between shots. The trade-off for the extra cooling is that the additional coolant has to be stored with the weapon, increasing the overall weight of the weapon.
Fusion Gun
The Eldar possess an unsurpassed mastery of heat-based weapons, and take savage delight in the devastation they create. They utilize a deadly Melta Weapon known as a Fusion Gun, by far one of the most common heat based weapons of the Eldar, it is a weapon that is unique to the Aspect Warriors known as Fire Dragons amongst the Eldar.
Though it can only be used at short range, it possesses a sophisticated targeting system, as befits high status troops like Aspect Warriors. Like most Eldar technology, the weapon is psychically activated, its resonant Wraithbone construction being sensitive to the Eldar's innately psychic mind. A Fire Dragons' Fusion Gun is linked to its targetter via its handle.
It is common knowledge to always replace your Eldar Flamer with a Fusion Gun as the Fusion Gun just does the Flamer's job so much better for roughly the same cost and effectiveness.
Fire Pike
A Firepike is a variant of the Fusion Gun used only by Eldar Fire Dragons Exarchs and the only Melta that is actually able to hit its target beyond 3 meters. Think of them as a elongated Fusion Gun that looks strangely phallic and erect. These weapons are sophisticated Melta Weapons marked by a distinctive long barrel that can project its deadly Melta beam a considerable distance further than the standard Fusion Gun. These potent weapons pre-date the existence of the Imperium of Man, having been created by Eldar artisans long millennia ago.
Operating similarly to most Melta Weapons, Firepikes project a lance of intense heat and radiation over a considerable distance that is capable of melting flesh and steel and carving through the toughest armour with deadly precision. Rare even amongst the arsenals of the Eldar, they are most often borne by the mightiest champions of their kind, relic weapons beyond contemporary means to reproduce even for the Eldar Craftworlds.
Inferno Lance
The largest known Eldar melta weapon and has one of the greatest range of all known meltas. Inferno Lances are a type of Eldar Fusion Weapon used by Skathach Wraithknights. A refinement of the same type of weaponry used by the Fire Dragons, the inferno lance is capable of burning through the armor of even the mightiest enemy war machine in a single, sustained burst of superheated particles and high-band radiation. To make the best use of the inferno lance, they are usually equipped as a pair for twice the dakka.
Its a shame it is not found as secondary weapons upon Eldar Titans such as the Revenant Scout Titan or the more powerful Phantom Battle Titan as this would give the Space Elves a good chance in poking a dent on the Imperium's Titans.
Dark Eldar Melta Weapons
Dark Eldar have the Heat Lance, a weapon that takes the best of the Lance and Melta weapons to obliterate any vehicle that gets too close.
Ork Melta Weapons
Ork Weirdboyz who have cheating dice rolls can throw a melty lightning blast capable of vaporizing Land Raiders. It's called Psychic Vomit. Yes, they vomit energy.
Necron Melta Weapons
Yep that's right, even the Necrons have Melta-Weapons in the form of the Heat Ray that can be used either as a focused beam (two shots at STR8 AP1) or as a heavy flamer (AP4 STR5) is used exclusively by the Triarch Stalker, and the short-ranged but very destructive Heat Cannon used by Sentry Pylons.
Tau Melta Weapons
Fusion Blaster
The primary Tau melta weapon is the fusion blaster, the equivalent of the Imperial meltagun, only with higher range (think middle point in between of regular meltagun and multimelta). It's always mounted on battlesuits or vehicles.
Fusion Collider
A nice new toy for the nice new XV-95 Ghostkeel suit, the Fusion Collider is functionally a Fusion Blaster with a Blast template allowing it to destroy tightly packed TEQ's and Vehicles.
Fusion Cascade
The close-quarters-focused XV-9 Hazard Battlesuit can also take a fusion cascade, which is capable of a higher rate of fire (it's Assault D3 instead of 1), at the cost of reduced strength (S6 instead of 8), though it is still as capable of penetrating armor as a regular Melta.
Fusion Cannon
Forge World produces a variant turret for the Hammerhead Gunship that mounts a twin-linked fusion cannon, which is the equivalent of the Devil Dog's Melta Cannon (i.e. a melta pie-plate) but twin-linked with two shots (though each of them counts as separate weapon, so no move and shoot both).
Fusion Eradicator
While humans made their titan-sized melta cannons having huge blasts, Tau went the different way and basically created gatling-multimelta, capable of evaporating any single target in a torrent of heat, and mounted it on Ta'Unar Supremacy Suit. They have yet to figure out how to make it more reliable, as in a current state it wears of so quickly the whole weapon system should be replaced after each battle, and it's not a cheap thing, but as the Imperial Knights lead the Imperium's counter-attack to retake the Damocles Gulf, a weapon, capable of reliably one-shoting them is worth any price.