from Wikipedia

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.

There are separate ad servers that publishers and third party (e.g. advertisers, marketers) use. Essentially, there is no difference in the technology that the ad servers provide, the key difference being the accessibility of data for optimized tracking and convenience. Advertisers and marketers use a centralized ad server that enables them to draw progress reports on demand and update their creative content in one place, rather than using individual publisher ad servers in which they will have to manage content across multiple servers with different publishers. Without this centralized hub which controls advertisers' rotation and distribution of content across the Web, there becomes issues around tracking and management of advertising material. If an advertiser had to make contact with each individual publisher whose ad server they are using, this would mean multiple sets of data to track and would also mean they need to update their creative content for each individual channel. This provides less-accurate, less-timely, and ultimately inconvenient results for advertisers. Publishers have separate ad servers to communicate advertising material across their domains only. This enables convenience for the publisher, as they will have access only to the advertising content they require for their publication rather than sort through an ad server containing all the advertising content in which Marketers/Advertisers are using.