A masonry heater warms space by storing the heat from occasional intense fires and gently releasing it into the space over a longer period of time. These have been used in various forms, right back to the early Roman hypocausts in which the smoke and flue gasses from a fire was passed under a floor raised on tile stacks.
Modern designs are based around a firebox core with heat exchange ducts or baffles constructed on a sturdy plinth to support the weight. A masonry shell is built around the ducting to form the thermal mass.
Masonry stoves may take 24 to 48 hours to reach full heat which makes them well suited for long term heating when compared to smaller metal stoves which supply rapid bursts of intense heat when lit. They are a good option if you are planning to provide the bulk of your home heating with wood fuel.