A kotatsu, used almost exclusively in Japan, 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.
A person then sits on the floor with their legs (or most of their body if napping) under the table with the blanket draped over the lower body.
Though this only heats up the lower body, it can keep one quite comfortable even in an unheated/under-heated room. In the summer, the blanket may be removed and the kotatsu can be used as a normal table.