An ad server is a computer server that stores advertisements and delivers them to website visitors. Ad servers come in two flavors: local ad servers and third-party or remote ad servers. Local ad servers are typically run by a single publisher and serve ads to that publisher's domains, allowing fine-grained creative, formatting, and content control by that publisher. Remote ad servers can serve ads across domains owned by multiple publishers. They deliver the ads from one central source so that advertisers and publishers can track the distribution of their online advertisements, and have one location for controlling the rotation and distribution of their advertisements across the web.

Another advertiser may only have paid to show one ad to each user per day. The ad server must therefore see if a user has seen that ad before, on that day and not serve it again if the user has seen it.

One aspect of ad serving technology is automated and semi-automated means of optimizing bid prices, placement, targeting, or other characteristics. Significant methods include:

• Behavioral Targeting - Using a profile of prior behavior on the part of the viewer to determine which ad to show during a given visit. For example, targeting car ads on a portal to a viewer that was known to have visited the automotive section of a general media site.

• Contextual Targeting - Inferring the optimum ad placement from information contained on the page where the ad is being served. For example, placing Mountain Bike ads automatically on a page with a mountain biking article.

• Creative Optimization - Using experimental or predictive methods to explore the optimum creative for a given ad placement and exploiting that determination in further impressions.

Ad Serving technology is the most complex when it is used by an Advertising Network. An advertising network buys ads from many web sites and therefore acts like an advertiser user of Ad Serving. When the network buys ads, it tries to place ads on sites where they work best. However an ad network then sells its aggregated ad inventory to advertisers. When doing this, it uses its Ad Serving software as a web site does. In this case it tries to make the most money by running the ads that yield the highest returns for the network.