An ad server is a web server, that stores advertisements used in online marketing and delivers them to website visitors.
The content of the webserver is constantly updated so that the website or webpage on which the ads are displayed contains new advertisements - e.g. banners or text - when the site or page is visited or refreshed by a user.
Ad serving describes the technology and service that places advertisements on web sites. Ad serving technology companies provide software to web sites and advertisers to serve ads, count them, choose the ads that will make the website or advertiser most money, and monitor progress of different advertising campaigns.