The answer to 'difference between tankless water heater and tank water heater' is that tankless water heaters heat water on demand, while tank water heaters store hot water in a tank. Tankless models are more energy-efficient and take up less space, but generally have a higher upfront cost. Tank heaters have a lower initial investment but use more energy to constantly maintain the water temperature in the tank.
Tankless water heaters only heat water when it's needed, which can save on energy costs compared to a tank model that keeps a full tank of water hot at all times. They also take up less space since there's no storage tank. However, tankless models typically have a higher purchase and installation price. Tank water heaters are generally less expensive to buy and install, but they use more energy to keep the tank of water heated.