This info mainly pertains to 2 strokes at this time but may be updated at a later date. Reading your spark plugs is a valuable tool for performance tuning as well as for identifying possible problems with the mechanical condition of your engine. Below are descriptions with pictures of different plug appearances and their causes. Later I'll be adding a section on plug reading as well.
Normal Appearance - This is a perfect plug. The color is mocha brown so the carb jetting is optimum.
The first three threads are black signifying the plug's heat range is matched to the application.
Wet Fouled - This plug is wet fouled. The spark plug's heat range is either too cold or the carb jetting is too rich.
Heavy Carbon - This plug has heavy carbon build up, possibly due to a bad crank rotary valve seal or a faulty oil injection pump.
Excessive oil has entered the combustion chamber, burning off and accumulating the carbon deposits on the plug electrode and insulator.
Engines showing these plug symptoms will have excessive amounts of blue exhaust smoke and may
also actually have oil being expelled as well if the crank rotary seal has failed.
Aluminum Fouled - This plug has tiny deposits of aluminum packed around the insulator.
The engine is suffering a meltdown from detonation or another internal failure.
With detonation, excessive heat can't transfer from the spark plug fast enough and the center of the piston melts causing the molten aluminum to collect on the plug.
Obviously, you should not operate an engine that shows those plug symptoms until you investigate the cause and correct the problem!