I was referring mostly to display advertising, but with video ads, VAST compliant players sent http requests to whichever endpoint specified based on different quantiles (start, 25%, 50%, 75%, video finish). This endpoint would be our servers so we can track which video and how much you've seen. The same is true for display, where we get an impression. You load the image which hits our ad servers, along with your cookie (who are you), and redirects to the actual image.