A wood biomass boiler is an appliance that can burn biomass fuel through a process called gasification, to heat large amounts of water in a tank wrapped around the combustion chamber.
Sounds simple, but to be truly efficient, a number of governing controls and pumps are needed to automatically regulate the air for combustion and the flow of water around the water jacket.
The heat generated is then stored in a large well insulated container called an accumulator or buffer. This allows the boiler to start burning and run for a long period ensuring that it gets to maximum efficiencies and can store all the heat generated for later use (rather like a car going on a long drive will produce better fuel economy than doing many short journeys).