Google knows what you searched for as well as your activity on partner web sites that use its ad services. If you use the Chrome browser, it may know every web site you've typed into the address bar. It may have all of your e-mails, your appointments and what you're watching (YouTube). It may have transcripts of your telephone messages (Google Voice). It may hold your photos in Picasa Web Albums, which includes face-recognition technology that can automatically identify you and your friends in new photos.
Technically Google doesn't know anything about you. But it stores tremendous amounts of data about you and your activities on its servers, from the content you create to the searches you perform, the Web sites you visit and the ads you click.
Google collects all data about your friends for the optimization of Google social search.
Google Social Circle page will show you all your connections split downs into various categories.
The Google gathers all this information from your Gmail Contacts, Gtalk chat list, Picasa, Google Profile, people you are connected through social networks, people you are following on Google reader and Google buzz.
