Pre-ignition is when the electrical spark is not the start of the flame front. It often but not quite always leads to knock. Even if it somehow is not knock is still bad because pressure builds before the piston is ready to convert the pressure to useful work.

Knock is a sudden explosion - detonation - rather than a smooth controlled burn. Instead of building, pressure spikes so fast it's like a hammer striking the piston or the cylinder or the valves.

Depending on the amount of air & fuel involved in the knock event as well as when it occurs, has an effect on its acoustics, but it always sounds like some kind of metal on metal.