This section of the Help gives details on what each piece of Airfoil does. Click on one of the sub-sections to the left to jump to the specified window.
Main Window - The basic control window of Airfoil contains all its key settings.
Effects and Radio Windows - Airfoil's secondary control windows allow for audio adjustment and control of USB radio devices.
Other Windows - Information on the other minor windows in Airfoil.
Menu Reference - Explains what each and every menu item does in obsessive, boring detail.