Just to try to define detonation:

When the flame kernel expands, the radiant heat combined with the compression heaating/pressure, cause a deisel like compression ignition of the fuel at the opposite part of the cylinder. This again recreates the problem, and what you have is a explosion instead of a burn. On a pressure graph youll see 300 times the peak pressure.

Preignition is just the hot metal in a cylinder causing a kernel to intiate before the spark, than the spary ignites, and can cause colliding flame fronts, either causing detonation, or just a lack of power.