Matki, (or Matka) is the Hindustani word for an earthen pot. It is used all over India, as well as Pakistan, Bangladesh and Sri Lanka, as a home "water storage cooler". It has been in use since ancient times and can be found in houses of every class.
The cooling process works through evaporative cooling. Water seeps from mini-pores in the pot and evaporates, thus making the water inside cooler than the outside temperature. Hence it is used only during summers and not in winters.