Publishers, advertisers and everyone in the middle

If only the advertiser and the publisher were involved, advertising would be simple. In reality, between a brand's budget and a website's bank account there can be half a dozen companies, each with a job and a fee. This lesson introduces the whole cast, so the rest of the academy reads like a story instead of alphabet soup.

Think of buying a house. You have a buyer and a seller, but also a buyer's agent, a seller's agent, a listing website, a bank, a surveyor and a lawyer. Each helps the deal happen, each takes a fee, and if you do not know who they all are, you cannot tell whether you are being overcharged.

The sell side: people with space

A publisher owns the audience and the ad slots: The Guardian, Times of India, Nikkei, Globo, a weather app, a Roku channel. Big publishers have their own sales teams and an ad operations team that sets up campaigns and keeps the plumbing working. Smaller ones rely almost entirely on automated tools.

Their most important tool is the publisher ad server. It is the traffic controller for every slot on the site: it knows which campaigns were sold directly, which must be delivered today, and when to open the slot to outside buyers. Google Ad Manager is by far the most widely used, which is one reason it sits at the centre of the US antitrust case against Google.

Next comes the SSP, or supply-side platform. The SSP is the publisher's automated sales agent: it takes each available impression, packages information about it, offers it to many buyers at once and tries to get the best price. Well-known SSPs include Magnite, PubMatic, Index Exchange, Google's AdX and Equativ.

The buy side: people with money

An advertiser is the brand that pays: Unilever, Toyota, Nubank, a local bakery. Big advertisers usually hire an agency to plan and buy media. Most of the world's largest agencies belong to a few holding companies: WPP, Publicis, Omnicom (which completed its acquisition of IPG in November 2025), Dentsu and Havas. Inside agencies, a trading desk is the team that operates the buying software.

That software is the DSP, or demand-side platform. A DSP lets a buyer describe who they want to reach, set budgets and prices, and then bid automatically on millions of impressions across many sellers. The Trade Desk, Google's Display & Video 360 and Amazon DSP are among the biggest.

The advertiser also runs its own advertiser ad server (for example Google's Campaign Manager 360 or Flashtalking), which stores the actual ad files, tracks every impression and click, and gives the brand an independent count to check against what sellers report.

The meeting place

An ad exchange is the digital marketplace where SSPs and DSPs meet and auctions happen. Today most SSPs run their own exchange, so the words are often used interchangeably. Before exchanges, the main middlemen were ad networks, which bought leftover space from many sites and resold it in bundles, often without telling buyers exactly where the ads ran.

The referees

Because nobody fully trusts anybody else's numbers, a whole category of ad verification companies checks the work. They measure whether an ad was actually viewable, whether it appeared next to safe content, and whether the "person" who saw it was a human or a bot. Industry bodies like the MRC in the US accredit these measurements, and groups like TAG run anti-fraud certification programmes.

Following one ad through the cast

1. BriefA sportswear brand tells its agency it wants runners aged 18 to 35 in Germany and India.
2. SetupThe agency's trading desk builds a campaign in its DSP and uploads the ad files to the advertiser ad server.
3. Supply appearsA running blog in Pune loads a page. Its publisher ad server finds no direct deal and passes the slot to its SSPs.
4. AuctionThe SSPs send a description of the impression to many DSPs. The brand's DSP bids, and wins.
5. Delivery and checkingThe ad loads, the ad servers on both sides count it, and a verification tag confirms it was viewable and human.
6. SettlementWeeks later, money flows from brand to agency to DSP to SSP to publisher, with each keeping a slice.

Some companies play more than one role. Google operates a DSP, an exchange and the dominant publisher ad server all at once, which a US federal court found in April 2025 was used to illegally maintain monopoly power. Amazon runs both a DSP and its own publisher tools. These overlapping roles are why "who works for whom" is one of the most important questions in ad tech.