A kotatsu is a wooden support covered with a heavy blanket or futon, on which the tabletop rests. The table is equipped with a heat source installed underneath it. Traditionally, Kotatsu have tended to be much more functional than elegant, which may not be the best choice for year-round use. More recently, as the heating mechanisms have become smaller and more hidden in the design, Kotatsu have started to become real competitors to the low table. You'll generally pay a little more for a Kotatsu, but given all you'll get in terms of a heating solution that can also be used year-round, they're definitely worth considering.