The term Boiler may refer to an appliance for heating water. Applications include water heating and central heating.
Boiler (water heating) Wikipedia
The boiler heats water to a temperature controlled by a thermostat. The water then flows (either by natural circulation or by a pump) to radiators in the rooms which are to be heated. Water also flows through a coil in the hot water tank to heat a separate mass of water for bathing, etc.
A back boiler is a device which is fitted to a residential heating stove or open fireplace to enable it to provide both room heat and domestic hot water or central heating.