The kotatsu is a low table that is covered by a kakebuton . Underneath it is equipped with a heater, nowadays nearly always electric.
People sit around the table, with their feet tucked under to take advantage of the heat.
This is the most common system of heating in the more temperate parts of Japan. One of its advantages is that it does not use much energy. In the colder parts of Japan, the traditional heater is called an irori . The irori heats the whole room but, in keeping with a people who sit on the floor, is built not into the wall but into a sunken part of the floor. Just as Japanese are using fewer tatami and more chairs and beds, kotatsu and irori are being replaced with kerosene, gas and electric heaters.