A wood burner works by allowing for a controlled area within which wood can burn. Wood is placed in the firebox and lit. Vents on the stove allow for an airflow that controls the speed of the burn and therefore the amount of heat.  Airflow on wood burners comes from the top of the unit, the reason being that wood will always burn better on a bed of ash as it draws air in from above.
Heat is then radiated via the metal body of the stove. Due to the closed front, you get a more efficient heat source than if using an open fire thanks to the gasses produced by the burning wood staying within the stove rather than dissipating into a wider space.