The way we get hot water has been the same for many years. We all have seen the water tanks in our closets or garages. And we especially remember them when they break. Natural Gas is used to warm the water in the tank, which we use for our faucets and showers. But they can be innefficient, because they can loose heat. Also, the heater is working to maintain that heat, burning gas and using energy, even when no one is home or using hot water.
A smarter way is to use tankless water heaters. This will heat the water only on demand when needed. So no gas is being wasted when no one is using hot water or when no one is home. And municipalities also give rebates for such efficient appliances. This saves us gas and energy, and in the end, less fossil fuels and CO2 is emitted in the air.