Air handlers and furnaces aren't usually found together in Tempe. If you rely on a furnace, you most likely shouldn’t need to spend time thinking about an air handler.
Air handlers are usually used with heat pumps and manage airflow throughout your house. Some systems also have backup heating and cooling components to assist the heat pump.
A furnace works in a different way by blowing warm air into your vents, which circulate it across your house. Since furnaces depend on combustion chambers to create hot air, they don't need some of the pieces you'll see in a standard air handler.