Air handlers and furnaces aren't typically found together in Tempe. If you use a furnace, you most likely won’t need to consider an air handler.
Air handlers are usually used with heat pumps and manage airflow throughout your house. Some systems also have extra heating and cooling components to assist the heat pump.
A furnace works in a different manner by blowing heated air into your vents, which circulate it around your house. Since furnaces depend on combustion chambers to create heat, they don't use some of the components you'll spot in a regular air handler.