At the most basic level, ad networks pool inventory of unsold ads from publishers and sell it to advertisers. They earn money by taking a cut of ad revenue, sometimes marking-up inventory before selling it. Ads are delivered to a publisher’s site by an ad network’s ad server via code on the publisher’s site that calls the ad. Performance is tracked via a tracking pixel from the ad network that the advertiser places on the conversion page(s) such as a thank you page on its site . The ad network’s ad server powers ad targeting, tracking and reporting on the campaign. Unlike AdSense where advertisers and agencies manage bidding, targeting and optimization themselves, many ad networks manage campaigns on behalf of agencies and advertisers. In these cases, an ad network and buyer negotiate the terms of an ad buy such as audience targets, impressions (the number of times an ad is served) and average cost per impression (CPM). The ad network then executes the targeting, optimization and reporting on the campaign.