About 7 years ago I was looking at a web page next to a real life friend who was also looking at the same page. Contents matched but those ads next to it that Google serves up were somewhat different. Digging in, I found the answer to be in Google's free analytics service. You see, when Google places free analytics on a website (to see visitor traffic, who is coming to see you, how they go through your site, etc.) they capture all kinds of browsing information. They traded a very good service that most people charge for to obtain tons of information on web use. And you know that drop down window that asks (on many sites) registration (email address mostly), well Google in their wisdom started the practice of matching up your email to your site visits and not so amazingly they matched up where you appear online via email registration with you IP address and browsing habits. Then they did a huge "sort" on you and out you come, a friend of Google and over time, they have you six ways from Sunday.
Americans spend about 1/2 an hour surfing daily (on average) and at about a page a minute. Do this for 10 years or so and you can imagine the details they have, like them or it or not. Google is ethical and the people there are smart and on a mission to maximize profits and provide services that, at the inception or conception of the Internet, could only be dreamed about. But they have you usually about 140 columns wide with everything from your sweater size to your choice of golf clubs (xxl and hogan's). It was all there so they must be my friend.