A heater is an object that emits heat or causes another body to achieve a higher temperature. In a household or domestic setting, heaters are usually appliances whose purpose is to generate heating (i.e. warmth). Other types of heaters are Ovens and Furnaces.
Heaters exists for all states of matter, including solids, liquids and gases and there are 3 types of heat transfer: Convection, Conduction and Radiation.
The opposite of a heater (for warmth) is an air cooler (for cold) (see air conditioning) used to keep the user cooler than the temperature originally surrounding them.