Air handlers and furnaces aren't often used side by side in Toronto. If you use a furnace, you probably don't need to consider an air handler.
Air handlers are often paired with heat pumps and manage airflow throughout your house. Some units also have secondary heating and cooling pieces to assist the heat pump.
A furnace runs differently by sending warm air into your vents, which move it across your house. Since furnaces rely on combustion chambers to produce heat, they don't have some of the parts you'll spot in a normal air handler.