Okay here we go........
Knock is a post spark event, although it is often easier thought and explained as pre spark. (Sometimes i explain it as pre spark as its easier to understand, but since ive started this journey......)
Pre ignition is a pre spark event. It melts stuff. It will likely only happen once and you will only know after it happens when your engine is in little melted bits on a trail back down the road that ends at your exhaust!
Pre ignition is caused by excess heat build up (and i am talking about LOTS of heat). Here the process is exactly as Nev describes. If you imagine when you compress something it gets hot. For example the entire need for intercooling. Imagine how hot it gets when you are compressing the mixture, and it spontaneously explodes, and you try to compress it some more! It builds loads of heat and melts stuff instantly.
Knock is a post spark event. Its caused when two flame fronts collide in the bore. The first is the nice burn from the spark plug which moves about 300 feet a second. The second is a spontaneous ignition caused by the heat generated during compression and trigered by the heat increase of the spark flame front. This explosive flame front (often multiple) moves at 3000 feet a second. The two (or more) flame fronts smash into each other with huge force and cause the damage that we asociate with knock. (Ie broken things!)
As to what knock sounds like, here is a relly good example.
http://www.phormula.co.uk/Player.asp...Analyser%20Pro
its a little advert for phormula products, but it demonstrates the sound that you hear when an engine is knocking. All engines sound slightly different, but the type of sound is typical.
Cheers,
Ben.