Air handlers and furnaces aren't typically found side by side in Memphis. If you use a furnace, you most likely don't need to spend time thinking about an air handler.
Air handlers are often used with heat pumps and deliver airflow around your residence. Some models also have extra heating and cooling pieces to assist the heat pump.
A furnace runs differently by blowing heated air into your vents, which circulate it around your home. Because furnaces use combustion chambers to make heated air, they don't use some of the pieces you'll find in a regular air handler.