Tankless Water Heaters heat water directly without the use of a storage tank. Therefore, they avoid the standby heat losses associated with storage water heaters. When a hot water tap is turned on, cold water travels through a pipe into the unit. In an electric Tankless Water Heater an electric element heats the water. In a gas-fired Tankless Water Heater a gas burner heats the water. As a result, Tankless Water Heaters deliver a constant supply of hot water. You don't need to wait for a storage tank to fill up with enough hot water.
Typically, gas-fired Tankless Water Heaters will produce higher flow rates than electric Tankless Water Heaters.