Air handlers and furnaces aren't typically found together in East Syracuse. If you rely on a furnace, you most likely don't need to think about an air handler.
Air handlers are frequently used with heat pumps and control airflow around your home. Some systems also provide backup heating and cooling components to help the heat pump.
A furnace heats differently by blowing hot air into your vents, which circulate it across your house. As furnaces have combustion chambers to produce heated air, they don't use some of the pieces you'll find in a regular air handler.