Programmatic
Ad server
An ad server is the software that stores ads, decides which ad to show in each slot based on rules and priorities, delivers it, and counts impressions and clicks.
An ad server is the decision-maker and scorekeeper of online ads. When an ad slot asks for an ad, the ad server checks every eligible campaign (dates, targeting, priority, frequency capping, pacing), picks one, sends back the creative, and logs the impression and any clicks.
There are two kinds. A publisher ad server (such as Google Ad Manager) manages a publisher's inventory and decides between direct-sold advertising campaigns and programmatic demand. An advertiser ad server (such as Google Campaign Manager 360 or Flashtalking) hosts the advertiser's creatives, tracks delivery across many publishers, and is often the count used for billing.
Because two ad servers usually count the same impression, their numbers rarely match perfectly, which is called a discrepancy. Ad servers also integrate ad verification and invalid traffic filtering. Google's dominance of publisher ad serving was central to the United States v. Google ruling.
Think of it like this
An ad server is like a restaurant's head waiter who decides which reserved or walk-in party gets each table and writes every seating in the book.
An example
A homepage has a guaranteed campaign at priority 4 and a programmatic line at priority 12. The ad server gives the slot to the guaranteed campaign until its daily goal of 200,000 impressions is met.
Related terms
Publisher ad server
A publisher ad server is the system a website, app or streamer uses to manage its ad inventory and decide, for every ad slot, which campaign or programmatic buyer gets the impression.
Advertiser ad server
An advertiser ad server is the system brands and agencies use to host their ad creatives centrally, serve them into publishers and DSPs, and track delivery and results across all of them.
Ad request
An ad request is the signal a web page, app or video player sends to an ad server or ad platform asking for an ad to fill a specific slot.
Discrepancy
A discrepancy is the difference between numbers reported by two systems for the same campaign, such as a publisher's and an advertiser's ad server counting different impressions or clicks.
Impression
An impression is one instance of an ad being served and counted as displayed to a user; it is the basic unit that most online advertising is bought and sold by.