An ad server is a Web server that stores advertising content used in online marketing and delivers that content onto various digital platforms such as Websites, social media outlets and mobile apps.

An ad server is merely the technology in which the advertising material is stored and is the means of distributing that material into appropriate advertising slots online. Ad serving technology companies provide software to Websites and advertising companies to serve ads, count them, choose the ads that will make the Web site or advertiser the most money and monitor the progress of different online advertising campaigns.

The purpose of ad serving is to deliver ads to users, to manage the advertising space of a Web site and, in the case of third party ad servers, to provide an independent counting and tracking system for advertisers/marketers.

Ad servers also act as a system in which advertisers can count clicks/impressions in order to generate reports, which helps to determine the return on investment for an advertisement on a particular Web page.