The three main types of furnaces are gas, electric, and oil. Gas furnaces are the most common type and use natural gas to heat air, which is then circulated throughout the home. Electric furnaces use electricity to heat air, which is then circulated throughout the home. Oil furnaces use oil to heat air, which is then circulated throughout the home. Each type of furnace has its own advantages and disadvantages, so it is important to consider the cost, efficiency, and safety of each type before making a decision.