Air handlers and furnaces aren't often found in the same home in Dallas. If you rely on a furnace, you likely don't need to spend time thinking about an air handler.
Air handlers are usually used with heat pumps and manage airflow around your house. Some units also have backup heating and cooling components to support the heat pump.
A furnace heats in a different way by pushing warm air into your vents, which circulate it around your home. Since furnaces use combustion chambers to produce hot air, they don't have some of the components you'll find in a normal air handler.