The Airloop is the HVAC system, which usually serve multiple zones, has a fan and cooling/heating coils, and operates either using a Thermostat/humidistat or air temperature/moisture set point control. See AirloopHVAC object.

The AirflowNetwork is a pressure/airflow model that is used to simulate building air flow in more detail. You would usually not use AirflowNetwork to initially create and simulate the building. When everything is set up and working building but then could use AirflowNetwork could be used to simulate the building in more detail, detail if needed.