EMP Warhead is pretty much a simple atomic warhead, however devised to be as 'dirty' as possible - causing en electromagnetic pulse as strong as possible, without the regard for accompanying radiation or the strenght of explosion itself. It can be used as both offensive and defensive 'defensive' warhead. In the offense, its explosions can temporary blind the enemy defenses, essentially wiping out ECMs and damaging the 'physical' AV defense, but it requires good timing to avoid blinding your own missiles. It is often called Shock and Awe missile doctrine. In defensive, EMP warheads can be used to detonate amidst the incoming missiles, blinding them, potentially shutting down or at least preventing them from evasive maneuvers for a while.