What is the difference between heat rash, heat stroke and sun poisoning?

All quite different.: Heat rash (or prickly heat, or miliaria) is the rash produced when heat causes your sweat glands to plug up. Heat stroke is the overheating of your entire body when you spend too much time in a hot environment, become dehydrated, and can't produce enough sweat to cool your body. Sun poisoning is just another name for a bad sunburn.