These are water heaters for domestic water, meaning for heating your home's drinking, washing and bathing water.
Tankless water heaters, also known as on-demand or continuous flow water heaters, only heat the water when needed. When water flow is detected the water heater turns on, and when the demand for hot water ceases the unit shuts down to conserve energy.
Heat pump water heaters heat the water with a highly efficient electric heat pump. A heat pump water heater is 3-5 times more electrically efficient than a traditional straight electric water heater.