Google has every e-mail you ever sent or received on Gmail. It has every search you ever made, the contents of every chat you ever had over Google Talk. It knows every Google Alert you've set up. It has your Google Calendar with all content going back as far as you've used it, including everything you've done every day since then. It knows your contact list with all the information you may have included about yourself and the people you know. It has your Picasa pictures, your news page configuration, indicating what topics you're most interested in. And so on.
If you ever used Google while logged in to your account to search for a person, a symptom, a medical side effect, a political idea; if you ever gossiped using one of Google's services, all of this is on Google's servers. And thanks to the magic of Google's algorithms, it is easy to sift through the information because Google search works like a charm. Google can even track searches on your computer when you're not logged in for up to six months.
Here is an interesting but disturbing link.