Originally posted by Bob Parmenter
Pre-ignition defines the air fuel mixture being ignited before the spark plug fires, or prematurely. The fuel mixture may burn normally rather than detonate, thought since it's occuring too soon in the timing, the heat of compression may cause detonation.

Run on is the engine continuing to have a fuel source to the combustion chamber, after the spark ignition source is turned off. The ignition source will be a chunk of deposit material, or a hot spot, or perhaps overly high combustion chamber temps. Again it may have normal combustion, incomplete combustion (most likely), or possibly detonation in severe situations.
Thanks, I appreciate it.