Well,
It is pretty simple. You have a banner made and submit it to the exchange. The exchange is group of websites that have agreed to exchange ads on areas the page owner determines. The page owner inserts a piece of code in his page that will display the banner. The banner is displayed that is partially random and partially base don te number of views each page provides for the other members. For example, if site A gets 100 hits and site B gets 200 hits, site B will get more banner exposed on others sites because they are providing more banner exposure for others. This is true, but if each site have equal popularity, then banners are random.
Members can check stats on their banners and how many times each banner provided hits to their site. You can also have multiple banners per site.
Neat process.
Chuck