It’s not so well known but there is a place where Google displays what it thinks about you – or at least what it is willing to tell you about it. If you think that Google knows everything, then you might want to visit this web page and check it out:
https://www.google.com/settings/u/0/ads/authenticated?hl=en
As for what Google has registered about me, the results were very-very poor: I had to uncheck 30 interests out of the 39 different interests Google has previously attributed to my Google Account. I must admit that as an online marketer perhaps I am not the easiest to profile since I am visiting a lot of different web pages, doing a wide variety of search queries, watching a lot of videos and ads, but still the original list was full of irrelevant interests like Dance & Electronic Music or TV Reality Shows or Billiards – topics I have never sought for.
As a frequent user of Google’s services like AdWords or Analytics, I tend to think that there are a whole lot of things Google knows about us statistically and builds many services upon that (such as conversion optimizer or similar audiences) without disclosing too much about its knowledge to us.
Perhaps I have overestimated Google’s ability to identify and categorize their users based on their activities? Once I was told by a Googler that cookie is the new keyword, but if there are so many irrelevant pieces of information attached to those cookies, there is still a long way to go.