I chose this adapter over its DVI brother as I wasn't sure if my external monitor supported DVI-D. The large end of the cable plugs into the VGA cable coming from your monitor - it's essentially an extension cable. Once connected the Mac recognises it straight away. Uncheck 'mirror image' and drag the open windows onto the external monitor for best performance. You can also choose where you want the menu bar and dock, either on Mac display or external monitor, regardless of where you have your open windows. All in all a great product.