The answer to 'difference between tankless water heater and tank water heater' is that tankless water heaters heat water on demand, while traditional tank water heaters store hot water in a tank. Tankless heaters are more energy-efficient and take up less space, but they have a higher upfront cost. Tank heaters provide a larger volume of hot water at once, but they use more energy to keep the tank heated.
Tankless water heaters heat water instantly as it flows through the unit, rather than maintaining a large tank of hot water. This on-demand heating is more efficient, as it avoids the standby heat loss of a tank. Tankless units also take up less physical space and can be installed in tight spaces. However, they have a higher initial purchase and installation cost compared to tank water heaters.