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, which is beneficial. 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. This is much more energy efficient than the traditional tank-style water heater which keeps all of the water in the tank hot at all times. Other benefits of tankless water heaters are that the hot water supply is endless so you'll never run out of hot water, and they take up much less space than a tank-style water heater.
There is an alternative use for these tankless heaters as well, which is to heat your home when combined with a hydronic air handler.