A tankless water heater directly heats water on demand, instead of maintaining a reservoir of hot water. Use this tag in conjunction with the [water-heater] tag
Tankless water heaters heat water only when you turn your hot water on. This typically saves money and energy because you're not constantly keeping a tank of water at a set temperature, regardless of how often you use the hot water. It also cannot run out of hot water (provided the energy and water flow endlessly).
Tankless water heaters can also be a great point-of-use device. Smaller electric devices fit under sinks so you can get hot water quicker. In some cases, you can even avoid having hot water runs to a location.