How Interest Categories Work
When a user visits a website, Google analyzes the content of the site and uses data from third-party companies to associate interests with a user’s cookie. Google takes into account how often users visit sites within those categories, among other factors, to assign interests to that user. Your ads will then show to users in the categories you’ve selected while they are surfing the web. If you are interested in what categories Google has assigned to you, you can find out by logging into your gmail account and going to www.google.com/ads/preferences.
Interest category marketing has been in beta for over a year, but Google announced today that this feature is now available to all AdWords advertisers. These categories allow advertisers to better target audiences across the display network based on their interests and demographics. You can choose from a list of categories that match the type of user you want to target, and Google will show ads to the people it thinks are interested in those categories.