Air handlers and furnaces aren't often found together in Birmingham. If you rely on a furnace, you likely won’t need to think about an air handler.
Air handlers are frequently paired with heat pumps and manage airflow across your residence. Some systems also provide extra heating and cooling pieces to help the heat pump.
A furnace runs differently by blowing heated air into your vents, which send it across your home. Since furnaces have combustion chambers to produce hot air, they don't need some of the parts you'll find in a normal air handler.