The most relevant heat-sources for saunas are the following:
Electric Stove: (classic sauna) These produce heat by electrical heating elements and pass it on to the surrounding air well as sauna-stones.
Infrared Radiator: These heat up the interior of the sauna-cabine by radiant warmth. The heat is supposed to directly warm up the body-surface of the user. There are two different subcategories in this field; ceramic- and carbon-emitters.