The right choice of plug can make all the difference to engine performance and reliability. The general 'rule of thumb' is: Hot engine = Cold plug and Cold/Cool engine = Hot plug.
So, if you have an older, slightly tired engine or you prefer to use straight fuel you should choose a hot plug. If you have a high performance, high revving 2 stroke that is working very hard a cooler plug is a good idea.
Always use four stroke plugs in four stroke engines, and they are also very good in inverted two strokes where starting and running at idle can be difficult.