A heater is a device for supplying heat, for example a radiator or a convector. These water heaters heat water only when it is needed. Turn on the heater and the interior of the car will soon warm up. A heater is a device for supplying heat, for example a radiator or a convector. Room heaters are used to heat a small space and are normally portable or fitted to a wall. Most room heaters use gas or electricity. Room heaters are convenient appliances that provide focused and localised heat which is particularly suitable in a room for people that are elderly, ill or with limited mobility. A heater is a piece of equipment which is used to warm a place or to heat water.