How do all these banner ads get placed and how does this work? First let's talk about what is an Ad Server, as per ZDNet's dictionary, an Ad server is defined as a "Web-based server that delivers banner ads and popup ads to the requesting Web pages. For companies that sell their own ads, the adserver may be an inhouse or co-located machine at an ISP, or it may be owned by an Internet advertising company".
The Ad publisher or the site where the ad will be placed contacts the Ad server to get the creative every time the page loads. Based on how the inventory for that particular placement has been sold, the ad server provides the particular creative to be published. This communication happens real time and sometimes slows down the page load. This is the main reason by sites like MSN, AOL do not want to use third party ad serving on their key homepage placements as they have over 100MM impressions in a day and if the page load is slow, they might lose consumers coming to their site.
The reporting part of the Ad server then captures the number of impressions, clicks, post-click activities, post-impression activities and all the other standard metrics based on how and what things have been tagged.