Air handlers and furnaces aren't often installed in the same home in Denver. If you use a furnace, you probably don't need to think about an air handler.
Air handlers are typically linked with heat pumps and manage airflow around your house. Some units also provide secondary heating and cooling parts to support the heat pump.
A furnace works differently by sending heated air into your vents, which circulate it across your home. Since furnaces depend on combustion chambers to make heat, they don't use some of the pieces you'll find in a standard air handler.