The differences between a gas, oil, and electric furnace are their fuel source, heating power, and operating costs.
Gas furnaces ignite propane or natural gas to heat and circulate air through the ductwork. Gas furnaces heat faster than electric.
Oil furnaces burn oil from an external tank to heat and circulate air through ducts. Oil furnaces are less efficient than gas and require more maintenance, but have faster heating.
Electric furnaces heat and circulate air with electricity or solar panels. Electric furnaces are the most energy-efficient, but have the highest operating costs.