When it comes to advertising, personal data can be incredibly valuable. And when it comes to personal data, few organizations know more about you than Google.
As you browse the web, Google tracks the sites in its sprawling ad network that you visit. As Google tracks your browsing patterns, it builds a profile of your inferred interests and demographic categories which are then stored in your Google cookie.
Here is how Google explains it: “For example, if a user browses many sports-related websites displaying AdSense ads or watches sports-related videos on YouTube, Google may associate a sports interest category with their cookie and show the user more sports-related ads. Similarly, if the sites that a user visits have a majority of female visitors (based on aggregated survey data on site visitation), we may associate the user’s cookie with the “female” demographic category.”
My cookie was quite accurate: it included valid generic categories such as music, business news, consumer electronics, enterprise technology, CRM, Air Travel, andMobile / Wireless. It also knew my gender and age band.
If you want to see what Google knows about you, you can view the contents of your cookie here.
Science of Revenue 
Good information. What does it say about you if “no interest or demographic categories are associated with your ads preferences so far.”
I don’t know! But there are probably a few possible explanations: (1) you have security preferences (or your employer does it if is a work machine) that limit data collection or use of cookies, (2) You don’t typically login to Google on the browser or device that you checked on, (3) you’ve recently deleted or reset cookies, (4) Google hasn’t observed enough data to determine patterns from their site network, or (5) Multiple people share your machine and so the patterns are not uniform enough to profile. But, I don’t know for sure! Thanks Sharon for the question!