Air handlers and furnaces aren't usually used in the same home in Jacksonville. If you rely on a furnace, you likely don't need to consider an air handler.
Air handlers are typically linked with heat pumps and manage airflow across your house. Some systems also have secondary heating and cooling components to help the heat pump.
A furnace heats differently by sending hot air into your vents, which move it across your residence. Because furnaces use combustion chambers to create heat, they don't use some of the components you'll spot in a regular air handler.