From the NYT:

Google agreed to its largest acquisition yesterday, reaching a deal to purchase DoubleClick, the online advertising company, from two private equity firms for $3.1 billion in cash, almost double what it paid for YouTube last year. And perhaps just as important, the deal kept DoubleClick from the hands of Microsoft.

For Google, the purchase is another step in its transformation from a search engine into an advertising powerhouse. DoubleClick, which is based in New York City, specializes in software for display advertising and has close relationships with Web publishers, advertisers and advertising agencies.

“It’s the two juggernauts in search and display getting together,” said Martin Reidy, president of Modem Media, an ad agency in the Publicis Groupe.

For computer users, the deal could mean more advertising tailored to their online habits, especially if DoubleClick and Google combine some of their huge databases of information on Internet use to direct specific ads to different Web surfers’ tastes. ...

Google, for all its outsize reputation, has made most of its money in the online basics: the text-based search engine and small text ads that are like the Yellow Pages of the online advertising. DoubleClick’s strength, by contrast, lies in flashy banner ads and, more recently, video ads that are more like high-end magazine or television ads. Google has taken steps in the last year to enter display advertising by expanding its AdSense program but has not gained great traction.

“Google really wants to get into the display advertising business in a big way, and they don’t have the relationships they need to make it happen,” said Dave Morgan, the chairman of Tacoda, an online advertising network. “But DoubleClick does. It gives them immediate access to those relationships.”