If you use Google’s search engine, 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-mail (Gmail), your appointments (Google Calendar) and even your last known location (Google Latitude). It may know what you’re watching (YouTube) and whom you are calling. 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.

And through Google Books, it may know what books you’ve read, what you annotated and how long you spent reading.

Technically, of course, 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, says Bankston, “is expecting consumers to trust it with the closest thing to a printout of their brain that has ever existed.”

How Google uses personal information is guided by three “bedrock principles,” says Peter Fleischer, the company’s global privacy counsel. “We don’t sell it. We don’t collect it without permission. We don’t use it to serve ads without permission.” But what constitutes “personal information” has not been universally agreed upon.

Google isn’t the only company to follow this business model. “Online tools really aren’t free. We pay for them with micropayments of personal information,” says Greg Conti, a professor at the U.S. Military Academy at West Point and author of the book Googling Security: How Much Does Google Know About You? But Google may have the biggest collection of data about individuals, the content they create and what they do online.