Air handlers and furnaces aren't often found in the same home in Dallas. If you use 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 offer extra heating and cooling components to support the heat pump.
A furnace heats in a different manner by sending heated air into your vents, which circulate it across your home. Since furnaces use combustion chambers to produce heat, they don't have some of the pieces you'll find in a normal air handler.