A kotatsu is a low, wooden table frame covered by a futon, or heavy blanket, upon which a table top sits. Underneath is a heat source, often built into the table itself. Kotatsu are used almost exclusively in Japan, although similar devices are used elsewhere.
There are two kinds of kotatsu used in Japan today, differing in the configuration and the type of heating:
Electric: The modern style of kotatsu (oki-gotatsu) consists of a table with an electric heater attached to the underside of the table. This evolved from a clay pot with hot coals placed under a table. The kotatsu is usually set on a thin futon, like a throw rug. A second, thicker futon is placed over the kotatsu table, above which the tabletop is placed. The electric heater attached to the underside of the table heats the space under the comforter.
Charcoal: The more traditional type is a table placed over a recessed floor (hori-gotatsu). The pit is cut into the floor and is about 40 centimeters deep. A charcoal heater is placed somewhere in the pit's floor, walls, or, as in the modern-style kotatsu, attached to the table-frame. There are pit type kotatsu with an electric heater too.