The basic parts of a furnace are a heating element to warm the air, an air handler to blow air through the ventilation ducts in the home and a thermostat to control the furnace's activity. A gas furnace also requires a ventilation system to exhaust combustion products outside the home.
The most common types of heating element are electric and gas, although oil-fired furnaces and boilers are still found in some areas. An electric furnace uses a resistance element to generate heat, requiring significant amounts of current to warm the home. A gas burner uses a pilot light or electric starter to ignite natural gas from the main line and a heat exchanger to transfer that warmth to the air flowing through the furnace. In each case, the thermostat switches the element on and off depending on the temperature of the air inside the home.
Most furnaces also include some sort of air filtration system. The simplest ones consist of a furnace filter, either fiberglass or cellulose media, inserted into the air intake system. This cleans the air of dust before it passes through the heating element, which can help reduce the risk of fire. It also cleans the air throughout the home as it passes through the system, reducing dust and allergens.