An ad exchange is a platform where publishers can auction slots for online advertisements, for which advertisers can bid, often facilitated by an ad network. For instance, after a user clicks on a link, the publisher may set up an auction for a banner on the webpage, using data about the user to enable effective targeted advertising. The auction will be concluded in the short period while the page is being rendered for the user, and the advert will appear instantly.
See also: BEHAVIOURAL ADVERTISING