The Three types of heat related illnesses are heat cramps, heat stroke and heat exhaustion.
All three of those heat related illnesses are serious and you need medical attention.
Heat stroke is the worst one of the three types of heat illnesses although like I said they're all serious.
To avoid heat related illnesses you should avoid staying out in the heat for too long and stay hydrated.
Always continue to drink fluids even when you're not thirsty.