Known also as Plasma Warhead Missiles, they deliver a payload of explosive plasma to the target.
Holding condensed plasma, when the missile hits the target the plasma is released in a violent explosion. That fact that it is a release of plasma also adds to the damage, adding more punch than a conventional missile. A few well placed Plasma Missiles could kill a weak Kaiju with little trouble.
Their appearance depends on the technology and the design used in making them.