wood burning heater/stove.
i've actually got one in my basement.
we don't use it too much unless it gets real cold but it helps keep the heating bill down during the winter.
the blocks along the sides of the chamber are fire bricks.
the tray at the bottom is for easily cleaning out the ash.
they say firewood warms you twice. i can't disagree with that since we chop, split, and stack our own.