Advertisers can choose from over 1,000 interest categories. Google says advertisers testing the capability were successful, citing one
advertiser increasing brand lift by 40%, and a shoe retailer driving 400% more conversions at a lower cost-per-sale. Pricing is on a cost-per-click model with an auction, Search Engine Land points
"The system looks at the types of pages that a user visits, considering how recently and frequently the person visits those sites, and associates that browser cookie with the appropriate interest categories," it writes. "Users can view and edit the categories they've been associated at Google's Ad Preferences page, and opt out entirely, if they so desire."