With it's ease of control and delivery, steam brought the advent of power to the shipping world. However it's time as passed, most ships nowadays use the more economical diesel or heavy fuel engines. Although boilers may no longer be commonplace for ship propulsion they are almost guaranteed to be one boiler for various duties on board a ship. Duties like heating cargo, fuel, and accommodations.
Some ships also use boilers for auxiliary power.
Such as deck winches and pumps, where electrical machines would prove to be a hazard as in the oil industry. Within the boiler, fuel and air are force into the furnace by the burner. There, it burns to produce heat. The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam. To the left is the basic theoretical design of a modern boiler.