Furnaces come in many shapes, sizes and brands. They also have different materials used to exhaust gases out of the home. Traditional, or conventional low to mid efficiency furnaces produce hot exhaust gases, and need metal vent pipes or chimneys to let these hot gases out. Modern, high efficiency furnaces produce much cooler temperature gases and can use plastic pipe materials such as PVC or ABS for the exhaust vents. Typically there will be a second plastic pipe as well that brings in fresh air.
What is an AC condensate line ? This line is an exit route for moisture collected by the evaporator coil as air passes over it. This coil is triangle shaped, with a drip pan at the bottom. It is out of sight and behind the ductwork just above your furnace. Its job is to remove humidity from the air and change it to water. This water collects in the drain pan, and is emptied out the condensate line down and out through the floor drain.