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 (rather like a car going on a long drive will produce better fuel economy than doing many short journeys) and can store all the heat generated for later use.
The wood burning appliances use fuel in the form of wood logs, wood pellets, wood chip or clean wood waste and there are products to burn each type.
A typical large commercial chip boiler installation.