I'ma expand on that a bit, 'cause it's all about learning, right?
Preignition happens when there's a hotspot in the combustion chamber that causes the fuel/air mix to ignite prematurely. Hence the name, pre-ignition. Because it's early, you get abnormally high combustion pressures in the cylinder. That can (but doesn't have to) cause enough of a heat buildup to cause detonation. So, preignition by itself typically won't hole a piston.
Detonation is when the fuel/air mixture cooks off all at once. There's no progressive flame front inside the combustion chamber... there's one big BANG... an explosion if you will. Combustion pressures go through the roof and sumpin's gotta give. Usually it's the piston that relieves pressure... in the form of a hole... right in the middle of it... because the crown is the weakest point.