semi metalic is just that, they have metalic particles embedded in the brakeing compound to prolong the age of them, as well as giveing you brake "feel", ceramics are made of a hard composite material, they are harder, last longer, and dont leave that dark brake "dust" on your rims, but………..they are so hard that they actually remove the hardened surface of the brake rotors prematurely, requireing the rotors to be changed sooner and more often, both are good, one is quite a bit more money then the other, {ceramics}

Ceramic brake pads: how long do they last?