Kilns are chambers used for heating.  They are used in processes such as baking bricks, curing, hardening, etc.  The kiln is a thermally insulated chamber which is heated.  Kilns are heated by an in-built furnace or by an external source, such as electric furnaces. 

In some kilns such as lime kilns and brick kilns, the material to be processed are exposed to the flames while in others the material is kept in a separate chamber which is heated. 

The kiln is usually lined with refractory bricks which can withstand high temperatures.