A short (hopefully simple) treatise on spark plugs *
puts on Professor-type voice and glasses *
All spark plugs get hot inside the combustion chamber when the engine is running - because there is burning going on. That heat needs to be controlled; if the nose of the plug (the bit inside the engine) gets too hot at high speeds, then its temperature may ignite the fuel/air mix before the advent of the spark ("pre-ignition") with very nasty effects. If it doesn't get hot enough at low speeds, then it builds up carbon deposits which short the plug out ("fouling") and stops the engine from running.
The terms "hot" and "cold" are comparative and refer to the rate at which the nose dissipates that heat. Crudely, you need "hot" plugs for "cold" engines (ie ones with lower power, not just running temperature) and "cold" plugs for "hot" engines (ie ones with higher power).
BUT: Spark plugs are VERY carefully selected by the manufacturers of both plug and engine, and unless you have made changes to that engine, don't mess about - you don't need to. If you do, and get it wrong, you can wreck the engine. However, if you have tuned the engine in some way, eg increased the compression ratio, advanced the ignition timing** etc etc - then you may require a "colder" plug to dissipate the heat faster, and stop the plug from overheating.
It is always safe to go colder - the worst that happens is it fouls up at low speeds; it is VERY rarely safe to go hotter. Ideally what you should do is ask the people who have done the mods/provided the equipment what THEIR recommendations are. Or ring up the spark plug manufacturer and seek their advice.
(** There has been a previous thread on the effects of advance, with some really good comments from someone whose name I've forgotten....
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