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
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.
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. 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.
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.