Otherwise known as a fusion gun, melter or a cooker, the meltagun family is a fairly common sight amongst the more technologically advanced armies of the Warhammer 40,000 universe. Essentially the name gives it away; the gun fires a blast of heat over a (very) short distance, turning just about anything unlucky enough to be in range into a molten heap within seconds.
The meltagun series (the meltagun itself, and its heavier counterpart the Multi-melta) are hampered by their shorter than average range. For example, the meltagun can only fire 12", half that of more common weapons such as the Bolter. The Multi-melta improves on this with a range of 24", yet this still pales in comparison to weapons like the Heavy Bolter (36") or the awe-inspiring Lascannon (48"). Yet this is easily compensated by the weapon's ludicrous strength, both versions of the gun only outclassed by said Lascannon.
They are also more effective at armour penetration than virtually any other weapon, even the Lascannon, due to their unique special rule. When within half their maximum range, a meltagun or Multi-melta may roll an extra D6 for penetration. This has given the weapons a reputation as tank-busters unequalled in the 40K universe.
Blood Angels players get a special treat, too. The leader of this chapter of the Space Marines, the venerable Commander Dante weilds a unique, pistol version of the meltagun, christened the Inferno Pistol. Although it may only fire a rather pathetic 6", it retains the values which make the other editions of the weapon so devestating.
Sources: The Warhammer 40,000 rulebook (3rd edition), and the accompanying book Codex: Blood Angels.