If you were wondering why Google bought DoubleClick, then look no further. It’s moving into the display advertising business.

Three principles underpin our approach to the display advertising field:

1. Simplify the system for buying and selling display ads: For example, our DoubleClick ad serving products help advertisers and publishers manage campaigns and ad formats across thousands of websites and from thousands of advertisers.

2. Deliver better performance that advertisers and agencies can measure: We're building a host of new features to help advertisers to run display ad campaigns across the Google Content Network (comprising hundreds of thousands of AdSense partner sites) and on YouTube. We're also developing better measurement and reporting technology so they can figure out what's working and what's not.

3. Open up the ecosystem: We want to democratize access to display advertising and make it accessible and open, like search advertising. We recently launched the Display Ad Builder to help businesses easily set up and run display ad campaigns. 80% of advertisers who use that product have never run a display ad campaign before.

We’ve been working hard to put these principles into practice, and today we're excited to announce the new DoubleClick Ad Exchange, a step towards creating a more open display advertising ecosystem for everyone. The Ad Exchange is a real-time marketplace that helps large online publishers on one side; and ad networks and agency networks on the other, buy and sell display advertising space.

If I ran an advertising or media agency, this would have ruined my breakfast. Most agencies made their money by having (or claiming to have) expertise in a highly inefficient and opaque marketplace. Suddenly, the game has begun to change.

The first casualty, though, (as the NYT points out) is likely to be Yahoo, which up to now had the display-ad exchange space almost to itself.