A boiler is a piece of equipment that converts a liquid into a vapour – most commonly, water into steam. Usually, it consists of a combustion chamber (furnace), heat exchange surfaces and expansion zones to allow the steam to expand. The fuel can be anything that burns, but commonly it is: natural gas, oil, coal, wood, or even garbage.

Fire-tube & Water-tube
There are two basic types of steam boilers: water-tube and fire-tube. In both designs, the goal is to heat water until it turns into steam.

A fire-tube boiler is basically a big pot of water with pipes running through it. Inside these pipes flow the hot gases from the furnace.	In contrast, the water-tube boiler is designed in reverse. The hot gases flow by tubes that are filled with water.
Fire-tube boilers are easier to design, install and operate and are used in small installations for such applications as heating buildings, factory processes and steam locomotives.	Water-tube boilers were designed to meet the need for higher pressure and temperature steam for use in steam turbines and power generation.

Modern boilers are made of alloy steel because they operate under extreme temperature and pressure conditions. Continuously adding heat to a closed vessel will increase the pressure inside that vessel. Eventually, the steel will fail, and you can have a nasty explosion. For this reason, boilers are always built with pressure relief valves to allow the steam to be vented if the pressure exceeds the normal operating conditions.

Let's take a look at a schematic of a power boiler.