A kotatsu (Japanese: 炬燵) is a non, wooden table frame covered with a futon, or heavy blanket, upon which a table shirt sits. Underneath is a heat supply, formerly a charcoal brazier nevertheless today electric, often assembled in to the table itself.  Kotatsu are used almost exclusively in Japan, although similar apparatus are used elsewhere: for example the Spanish brasero or Iranian korsi.
There are two Types of kotatsu used in Japan today, differing from the configuration and the type of heating:
Electric: The modern style of kotatsu (oki-gotatsu (置き炬燵)) consists of a table using an electric heater attached to the underside of the table. This evolved by a clay pot with hot coals placed below a table.  The kotatsu usually is set onto a thin futon, like a throw rug. A 2nd, thicker cushions is placed over the kotatsu table, above that the tabletop is placed. The electric heater attached to the bottom of the table heats the space below the comforter.
Charcoal: The more traditional type is a table placed above a recessed floor (hori-gotatsu (掘り炬燵)). The pit is cut into the ground and is about 40 centimeters deep. A charcoal heater is placed somewhere in the pit’s floor, walls, or, as from the modern-style kotatsu, attached with the table-frame. There is pit-type kotatsu using an electric heater too. Visit kotatsutable.org for more info.
There are similar systems in countries around the world: economical and often sociable ways to keep warm while sitting still. The Netherlands used to use a foot stove. From the First World War, British Royal Engineers built’Japanese footwarmers’ from the trenches. 
For centuries, a very similar thing called a sandali has been used widely in Tajikistan and Afghanistan.