Boiler

A boiler is a device used to heat water or other fluids until it boils or before boiling point. The heated or vaporized fluid is then used for various process applications, such as central heating, power generation, cooking, and sanitation. 
Hot water boilers heat water for the purpose of domestic or commercial heating and hot water supply.
Steam boilers generate steam in order to power turbines for power generation and various other industrial heating applications.
Fuel.
The source of the heat for boiler can be from coal, oil, wood, rejected heat or natural gas.
Types.
The different types of boilers find their use in different applications.
Steam heating.
Where steam is required for space heating, the boiler is often called a "steam generator". A niche application was for heating railroad passenger cars while steam locomotives were being phased out. The steam generator would be oil-fired on diesel locomotives or electrically heated on electric locomotives. Steam heating is now uncommon on trains. Modern trains are usually heated by electric heaters.