Introduction to Ad Serving

In traditional offline advertising, a company creates an ad campaign and then purchases advertising slots using different kinds of media, for example, on television, in magazines and newspapers, and on roadside billboards. With online advertising, the same thing happens, but the logistics behind media planning, or selecting where and when you will run your ads, are different. 

That's because the Internet is a much more fractured, individualized medium. There are far more websites run by far more organizations than there are TV networks and shows, and the interaction between consumer and media event has been narrowed down to a single person viewing a single webpage or using a single mobile app. Think about that versus a newspaper being printed once a day and read by thousands or a TV ad aired to a large population. Another large difference is the ability to tag consumers via their browser's cookie file or by their mobile device ID, which allows advertisers and other parties to segment consumers and collect data on ad effectiveness on an individual level.

Who Is Involved in Online Advertising?

Many different parties interact in the online advertising ecosystem:

Publishers, advertisers, and networks interact through unified ad trafficking systems called ad exchanges.  An ad exchange allows advertisers and publishers to use the same technological platform, services, and methods, and "speak the same language" in order to exchange data, set prices, and ultimately serve an ad.  Major ad exchanges include DoubleClick, OpenX, Facebook's FBX, and Xandr.

What Is Ad Serving?

Ad serving is the process of determining which advertisement goes in which ad slot on a publisher's webpage or mobile app and then delivering the advertisement.  The computer or group of computers that's responsible for this is called an ad server. Major publishers, networks, and advertisers sometimes have their own ad servers.  Most large ad servers also:

An ad server can be a publisher's ad server, where the publisher decides who gets the impression, or an advertiser's ad server, where the advertiser decides which creative goes in the slot they have been assigned. Or it can be an intermediary ad server that attempts to maximize the benefit to both sides. Some ad servers are small, in-house decisioning systems, and others are major ad servers used by many clients such as Xandr or Google's various ad serving products.

How Does Ad Serving Work?

In order to understand ad serving, it's important to understand the following concepts:

  • Placement - The slot on the publisher's page or location in an app where the advertisement will appear.
  • Creative - The actual advertisement.
  • Impression (imp) - A creative served to a single user at a single point in time.
  • Ad Tag - A snippet of code that requests or passes along data.  The data may be the size of the placement, the publisher's URL, or some other kind of information, or a creative or other content.  For more information, see Ad Tags.

In offline print advertising, publishers sell ad space.  In online advertising, publishers typically sell impressions – a combination of ad space and the number of times an ad displays. Payment terms are sometimes based on impressions, but they can also be based on conversions, or actions taken by end users, such as purchasing an item or signing up for a mailing list.

Determining which ad to serve depends on the advertiser and publisher requirements for logistics, targeting, advertising preferences, and pre-existing relationships.  An ad server may need to:

When targeting ads or evaluating bids, the ad server must take into account many considerations such as:

Further Reading

The above explanation is a conceptual version of ad serving, which can be a very complicated world with a lot of hops between a page loading and an ad being served. If you are new to ad serving and ad tags and all the hops that go into a single impression event, please see Ad Tags and the other pages below.