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Actually Google has much more information on you than what's on your Google account. They're an advertising company that makes money by tracking what you do on the web and showing you targeted ads. With online ads, the more tightly they target an audience, the more you get paid. So every scrap of data about your behavior is valuable to Google.

* Many, many sites have a script from "google-analytics.com" (happily not this one). This is a common service Google provides to website admins to gather statistics on their visitors, such as how many people accessed the site, what country they are from (can be easily found out from your IP), what site they came from (many sites put referrers in their links to transmit this information), what search keywords you used to find the page (if you found it from a search engine), what OS you run and so on. Of course, all this data passes through Google. They don't really say whether they do anything with it, but chances are they log all of it. Basically, if you go to any site using Google Analytics, they now have all the above information. Google Analytics isn't the only one, by the way - Yahoo, Microsoft, and many other companies have services like these.
* If you are logged in to your Google account, all your Google searches are logged. This includes how much time you spent looking at the search results, which results you click, even if you type something in the search box but then change your mind and delete it. They then use this to show you results they think you want to see.
* If you are not logged in, your searches are still associated with your IP. If you then, say, login to Gmail and check your emails from the same IP, Google can now easily consult their logs, see that the IP address searching for so and so terms has also logged in to this account, and deduce that you made those searches. For public computers shared by many users, the association is harder, but something like your home PC that only you use makes this trivial.
* If you use Chrome, even if you uncheck the "report statistics" options, it will phone home and tell Google what you do online, and even what you type in the address bar.
* If you have an Android phone, which uses Google's OS, it probably has Google software on it tracking where you are (it can figure this out by looking at the cell tower you are using or nearby Wi-Fi spots). Granted all phones do this.
* This is probably obvious, but they scan your all emails, Hangouts messages and so on and it to the profile they have about you.

There isn't really much you can do to see what Google has on you, since a lot of it they keep secret. The only option is to avoid using Google services as much as possible.

Besides this, user data is a hot commodity in the advertising business. Companies constantly track every single thing about you they can get away with. They then use it to show you targeted ads (these are more profitable for them), and sell these databases to each other.