Missile Launcher
The Missile Launcher is a very simple weapon, being little more than a tube capable of firing different kinds of explosive payload, similar to the Mortar. The Missile launcher is capable of firing a variety of payloads, chief amongst which are the anti-personnel Fragmentation missiles, and anti-vehicle Krak missiles. One of the main advantages of the Missile launcher is it's cheap manufacturing, ability to take several kinds of munitions, and superior portability.
Usage by the Imperium
The Missile Launcher is one of the most common explosive weapons fielded by the Imperium of Man, employed very commonly by Imperial Guard Heavy weapons teams, and extensively used amongst Space Marine squads of all kinds. It also sees use among the ranks of the Chaos Space Marines. It's incredible versatility in payload allows it to be a potent anti-infantry and anti-vehicle weapon when the situation calls for it. Its anti-vehicle capabilities are only outstripped by the Lascannon and vehicle-mounted weapons, and it is easy to learn and easy to use.
Back in the days of the Great Crusade, these things were the goofiest looking weapons ever. Dark Eldar laughed at how oddly shaped and impractical they were. Seriously.
Usage by the Eldar
Unlike the ones used by the Imperium, Eldar missile launchers consist of several pods of small missiles, using a cluster of small projectiles instead of a single large one to deal damage. Of note is that the number of openings on the launcher does not reflect the number of missiles that it is capable of holding: each firing tube is connected to several smaller chambers, in which the missiles are held and fired from.
Dark Reapers
The Dark Reapers aspect warriors use a variation on the Eldar Missile Launcher: the Reaper Launcher. Firing a hail of small missiles over a very large distance, they can do considerable damage to Space Marines and all but the largest of Tyranid organisms. Exarchs of the Dark Reapers can use either a regular Eldar missile launcher or the Tempest Launcher: a modified Reaper Launcher that shoots its projectiles in an arch, bombarding a large area with lethal projectiles.
Usage by the Tau
The Tau also use missile launchers, albeit only on their battlesuits and vehicles. The main difference between their launchers and the Imperium's is that their missiles all have guidance systems that allow them to Robotech around obstacles on the way to their target. They also receive a substantial increase in accuracy when their targets are illuminated by markerlights.
Da best rokkits (Orkz)
Those of us boyz who wanna crack open da humie metal boxes get da Rokkit Launcha an' da Tankhammer. Da Launcha got'z a button dat shoots rokkits at da tank. Even snots know dat. Tankbustas who like krumpin in melee can use da Tankhammer, its just a buncha 'splosives on a stikk. Hit da humie wagon an' it goes bang.