Air handlers and furnaces aren't usually installed in the same home in Hillsborough. If you use a furnace, you likely shouldn’t need to think about an air handler.
Air handlers are typically used with heat pumps and control airflow across your house. Some models also offer backup heating and cooling pieces to help the heat pump.
A furnace heats differently by pushing hot air into your vents, which circulate it around your residence. As furnaces have combustion chambers to make hot air, they don't use some of the parts you'll spot in a normal air handler.