whats the difference between them?

 

Drilled rotors are lighter than slotted rotors, but they are more likely to crack at the cross-drilled holes (holes are always stress-raisers -- of course, round holes are always better than square holes in this regard).

Holes or slots help brake performance by reducing the hot gas and brake dust buildup between the pad and rotor. The openings provide an escape route for all this junk. This is usually more of an issue on race cars with large brake pads (more pad surface area to trap stuff) than street cars with smaller pads.