## 2 Online Advertising Ecosystem--Key Components  
In this section, we introduce the main elements of an online advertising ecosystem, the critical interactions among these elements, and various technologies involved in delivering the right ads to the right people at the right time. As shown in Figure 1, an online ad system consists of three key players: publishers, advertisers, and users [1]. The interactions between these players are facilitated by an intermediate infrastructure--the so-called ad platform. **Publishers or sellers are the providers of ad system inventory (ad inventory or the amount of ad space in a publisher web page). They are the entities that create (and sometimes manage) the actual online content in a set of web pages [6]. Publishers receive money to put advertisers' content in front of online audiences by integrating advertisers' content into their web pages. Publishers' web pages are considered the users' entry-points into the ad ecosystem. **  
**Advertisers** are persons or companies that want to promote a product or a brand by displaying a related ad on third-party (i.e., publishers') websites. Advertisers generally aim to place their ads on popular high-traffic sites. **  
**Users** are the power engine of the ad ecosystem, as their requests for pages containing ads are what advertisers are looking for, and their actual views of (or clicks on) ads generate revenue for publishers. Furthermore, the data obtained by monitoring users' actions on a publisher's website will determine the specific location of various advertisements on the pages of this site. **  
**Ad Platform** is a framework that connects advertisers to publishers. Alternatively, it is a group of entities that facilitate matching the demand and the supply end of the ad ecosystem [6]. On one side, ad platforms assist publishers in finding the best match between their audiences and the advertisements they are supposed to showcase; on the other side, they help advertisers reach the most appropriate potential audience and customize the advertising material to them. The modern ad platforms are made up of a number of distinctive intermediate entities, including Ad network, Ad Exchange (AX), Supply Side Platforms (SSPs), Demand Side Platforms (DSPs), and Data Exchange. Each of these entities has unique responsibilities and mechanisms of operation; and, when all the entities work synergistically, they make the ad platform processes more effective, flexible, and transparent. The role of different entities comprising the Ad Platform is illustrated in Figure 2 and described below. Figure 1: The key components of online digital advertising ecosystem. Figure 2: Sub-elements of Ad platform in online advertising ecosystem. **Ad Network** emerged in the mid-1990s [7] as one of the first components (i.e., facilitators) of online advertising technology. They came into existence due to the fact that for many advertisers, running direct buys from publishers at high volumes was not sustainable. Hence, ad networks' purpose is to collect inventory from a range of publisher sites, segment it (e.g., based on geolocation, age, gender, interest, behavior of potential customers/users), and sell it to advertisers in advance and 'bulk'. In other words, Ad networks work with publishers to more efficiently sell ad impressions that they have not sold directly to advertisers. They also work with advertisers to develop coordinated and most effective ad campaigns across different sites. Examples of large ad networks include Google AdSense, Media.Net, BuySellAds, and Conversant. **Ad Exchange (AX)** is an open marketplace that auctions publishers' ad impressions in real-time among multiple bidding advertisers and sells them to the highest bidder. The whole process may take a millisecond. The key advantages of Ad exchanges (over Ad networks) are that they give advertisers better control, targeting capabilities, and monetary transparency. Namely, they allow advertisers to serve their ads to the right viewer, at the right time, and through the right publisher.