Air handlers and furnaces aren't usually found together in Livermore. If you use a furnace, you likely shouldn’t need to spend time thinking about an air handler.
Air handlers are frequently used with heat pumps and control airflow across your house. Some systems also have secondary heating and cooling components to support the heat pump.
A furnace works differently by sending heated air into your vents, which send it throughout your residence. As furnaces rely on combustion chambers to produce heat, they don't have some of the components you'll see in a normal air handler.