A heat wave is a prolonged period of excessive heat and humidity.
The heat index tells how hot it really feels when relative humidity is added to the actual air temperature.
Heat cramps are muscular pains and spasms due to heavy exertion.
Heat exhaustion occurs when people exercise heavily or work in a hot, humid place where body fluids are lost through heavy sweating.
Heatstroke is a life threatening. Your temperature control system, which produces sweating to cool the body, stops working.
Sunstroke is another term for heatstroke.