Air handlers and furnaces aren't typically found together in Mobile. If you rely on a furnace, you most likely don't need to spend time thinking about an air handler.
Air handlers are often linked with heat pumps and control airflow across your home. Some units also have secondary heating and cooling components to support the heat pump.
A furnace runs in a different way by sending warm air into your vents, which circulate it throughout your house. Because furnaces depend on combustion chambers to produce heated air, they don't use some of the components you'll see in a regular air handler.