A Heat Source is generally any object which emits enough heat to be used for Cooking most food items. The caveat is that some food items which are considered baked goods, such as breads, cakes, pies, etc., specifically require an oven for cooking.
To further complicate things, the various forges which may be used with Blacksmithing to craft items or smelt ingots may be used for cooking food, but not vice versa.
Additionally, most heat sources emit light, providing a pleasant glow in a home at night.
One of the most handy heat sources is the Heating Stand as it can be carried in a player's backpack but also be used for cooking some foods. This is especially helpful for fishers who may have large piles of fish but do not wish to haul it all to a conventional kitchen or tavern for cooking.