The integration will soon expand since Google intends to offer behavioral targeting or interest-based advertising, as Google likes to call it. "We think we can make online advertising even more relevant and useful by using additional information about the websites people visit. Today we are launching interest-based advertising as a beta test on our partner sites and on YouTube. These ads will associate categories of interest — say sports, gardening, cars, pets — with your browser, based on the types of sites you visit and the pages you view. We may then use those interest categories to show you more relevant text and display ads," explains a post from the Official Google Blog.
The DoubleClick cookie contains a unique ID that is associated with all your visited pages that include ads served by DoubleClick. If you're visiting a lot of pages related to music, Google will place you in one of the 600 predefined categories (most likely, music enthusiasts) and will use this information to show more ads about music. For now, Google will use interest-based targeting to show better ads when the content of a web page doesn't include enough information to serve contextual ads.
While behavioral targeting is not new and many other companies are using it, Google tried to alleviate the worries about profiling users: it won't create sensitive interest categories like race or religion and it won't cross-correlate the data with other information saved in Google accounts.
Google tries to use the enormous reach of the content network (75% globally) to attract more display ads, but the risk could be too high: Google's ads were perceived as non-intrusive, relevant and complementary to the page where they were placed. Focusing more on display ads, using recently visited pages to target ads to users could change that perception and Google will lose its most important asset: user trust.