[Pure_Insanity8]

Detonation is basically the fuel/air mixture igniting before it is supposed to. Usually "hot spots" occur somewhere in the combustion chamber, on the piston, etc.. During compression the fuel/air mix is being heated enough to where the hot spot can ignite it (before the spark plug fires). The flame front originating from the hot spot will usually end up "running into" the flame front created once the spark plug fires. So, of course, the mix is going to burn faster than it should, not to mention the violent collision of the two (or possibly more) flame fronts.
Even though under normal circumstances the burn for a single compression to power stroke is VERY fast (can't remember of hand but I think in the few thousandths of a second), this "unintentional" type of burn is going to start leaning closer to an explosion instead of a burn. When this "explosive" force is pushing down on the piston while it is coming up on the compression stroke... let's just say not cool.

I'm not the best at explanations... but I hope that gives you some idea of how the piston could be blown apart among other problems (like to your crankshaft or connecting rods).