Air handlers and furnaces aren't usually found together in Tempe. If you use a furnace, you likely shouldn’t need to spend time thinking about an air handler.
Air handlers are frequently paired with heat pumps and deliver airflow across your residence. Some models also have backup heating and cooling pieces to help the heat pump.
A furnace works in a different way by pushing hot air into your vents, which circulate it across your house. As furnaces have combustion chambers to produce warmth, they don't need some of the parts you'll see in a normal air handler.