Air handlers and furnaces aren't usually installed together in Boise. If you have a furnace, you likely shouldn’t need to think about an air handler.
Air handlers are usually used with heat pumps and control airflow throughout your home. Some models also have secondary heating and cooling parts to assist the heat pump.
A furnace works in a different manner by sending warm air into your vents, which move it around your residence. Since furnaces depend on combustion chambers to create warmth, they don't need some of the parts you'll see in a regular air handler.