The ad tech industry is waging a secret war on Google

DoubleClick for Publishers (or “DFP”) product has enormous market share among publishers using software to serve ads directly on their websites. Virtually everyone in the market uses DFP.

The indirect market, however, is fairly fragmented, with many exchanges and ad networks competing to win over publishers, including Google’s Ad Exchange (“AdX”). Google AdX has a unique advantage in the indirect market, however, because it integrates tightly with DFP.
Google allows DFP publishers to turn on a feature called “dynamic allocation” that lets AdX compete directly for impressions with the deals sold by the direct salesforce.

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With header bidding, third-parties like publicly-traded Criteo, or Amazon’s A9 ad unit ask publishers to put a special Javascript call in the header of their webpages. This header makes an ad request before DFP and figures out if there are any advertisers willing to pay for the impression and how much they want to pay.
In roughly 200 milliseconds, the header tag determines a price, then uses Javascript to pass the auction price into DFP to compete with all the ads the salesforce may have sold, as well as the AdX price. It’s a bit of a hacky workaround, but it works, and publishers will do anything that produces more pennies on each view of their page.
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The future of header bidding

In June of this year, AdExchanger ran an article describing the technique in depth, and suddenly implementation became widespread.
Important questions remain about the deployment of header bidding, including which demand sources to include, how many to include, whether publisher data is “leaking” to those bidders they can use elsewhere, and the effect of these additional calls on page load times.
http://www.businessinsider.com.au/how-ad-tech-companies-are-using-header-bidding-to-tackle-googles-dominance-2015-9