I am running a web application where I want to place context-sensitive ads in.

The content being displayed is private data of my users (not visible to the public), like email content and such. Thus, for privacy reasons, I don't want this information to be redirected to 3rd partys, such as Google servers.

Now, if I'd display AdSense ads, what exactly does this script do in order to display the ads?
Does it just parse the content of the site and then display relevant ads or does it actually send the content to Google servers in order to find the right ads to display?