Submachine guns are close-ranged, low calliber automatic weapons with a high fire rate. Sub machine guns are cheaper alternatives to assault rifles and are typically used in close quarter combat. Their low accuracy matched with high rates of fire and large ammo capacities make for excellent crowd control and great replacements for pistols early in the game.
There are only 3 of them in Dead Trigger. In Dead Trigger 2, the player can dual wield, though the Uzi is the only Submachine Gun known so far to do so.