Many get confused between the furnace, water heater, and boiler. All three serve the same purpose of providing comfort during winter, but the functioning of these three devices is different.
A furnace circulates warm air through ducts, a water heater provides hot water for daily activities, and the boiler generates steam and then circulates the heat via radiators.
A boiler is more energy-efficient and less noisy than a furnace system, but the boiler is also a bit more expensive and hard to install than the central heating system.