Traditional gas-powered tank water heaters have been one of the biggest energy consumers in your home. With hot water tanks, you have to pay to heat water you're not even using. With a tankless water heater, when you turn on your hot water tap, cold water travels through a pipe into the water heater unit. The gas burner in the unit heats the water, delivering a constant supply of hot water. You don't need to wait for a storage tank to fill up with enough hot water and you have an endless supply of hot water. 
Tankless water heaters are especially well suited when space is important. They can be wall mounted and take up significantly less space than hot water tanks. They can be installed in closets and are great for condominium and apartment units. 

In the combustion chamber of a boiler, natural gas is burnt to produce heat. The hot flue gases from the combustion process pass through a heat exchanger where the heat is transferred from the hot flue gases to the water that flows inside the heat exchanger. The heated water is then pumped through to heat your home and domestic hot water. In condensing (high-efficiency) boilers a second, or larger primary, heat exchanger removes even more of the heat from the flue gases. This pre-warms the water coming back into the boiler, so you burn less gas to heat the water up, making it more efficient. Because the flue gas loses more of its heat to the secondary heat exchanger the gases condense, hence the name condensing water heaters.