The hot/cold referance is to how much heat the plugs pulls out of the combustion chanber. It isn't that the plug is electrically hotter or colder.
The waterjacket is very close to the threads surrounding the sparkplug, a hot plug will have a small amount of mass and won't transfer heat to the waterjacket - if you look in the end of a 'hot' plug you will be able to see deep inside. A 'cold' plug will have a lot of mass and surface area, you won't be able to look down into one of them, they'll end right at the end of the threads. They pull heat out of the combustion chamber and ito the waterjacket.
The plugs needs only to be hot enough to keep itself clean at idle.
We can change exhaust temps by 150ish degrees by 1 full step in heat range, race plugs can come in 1/2steps for tuning.
You can cool down a warm engine in august with a simple change in the plugs' heat range.
