If water is heated for distribution as either steam or hot water, you have a boiler. If air is heated, you have a furnace.
In both cases, the burner heats a chamber around which either water or air circulates. Just like soup in a pot on a stove, never once does the water or air that heats your home come in contact with the actual oil or gas flame. This is a "closed system", which keeps your family safe. All emissions go straight up the flue, or chimney.