Spark Plug Heat Ranges
A Spark Plug's heat range is a measure of it's ability to conduct heat away from its firing end. A COLDER or 'harder' spark plug is more suited to a hotter running engine (i.e. high performance, high revving, tuned or driven very hard), the shorter heat path will conduct heat more quickly, ensuring that the tip does not overheat. Conversely, a HOTTER or 'softer' spark plug has a longer heat path and will maintain a higher tip temperature, carbon deposits are more likely to 'burn off' and the plug tip will stay cleaner.
This is a *basic* table of approximate spark plug heat range equivalents from manufacturers NGK, Denso and Champion. Note that Champion have further additional heat ranges for different application types (e.g. Marine engines).