Photo credit: Bored Panda
A kotatsu is basically 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. They are used almost exclusively in Japan, although similar devices are used elsewhere. The modern style of kotatsu 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 usually is 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. Continue reading for more pictures, another video and additional information.
Most Japanese housing is not insulated to the same degree as a western domicile and do not have central heating, thus relying primarily on space heating. Heating is expensive because of the lack of insulation and the draftiness of housing. A kotatsu is a relatively inexpensive way to stay warm in the winter, as the futons trap the warm air. Families may choose to concentrate their activity in this one area of the house in order to save on energy costs. In the summer, the blanket may be removed, and the kotatsu used as a normal table.