Not very. Web users leave behind footprints in server logs that record their activities. The logs show the IP address of a user's computer, the date and time a visitor clicked on a Web page, the user's PC operating system and browser, and the referring URL that brought them to a site.
An IP address can, with a reasonable degree of accuracy, be used to identify a user's location, through a geolocation service or an Internet service provider. When compelled by law or sometimes merely at the request of legal authorities, ISPs will identify their subscribers.
The referring URL is the link that brings a visitor to a Web page. When associated with a Web search, the referring URL typically includes the keyword used. Thus, when someone conducting a Google search for "informationweek" comes to InformationWeek .com, the referring URL--http://www.google.com/search?&q= informationweek--gets sent to our server.
What this means is that controversial keywords--say "bomb recipes" or "assisted suicide"--are transferred to the server at the end of the link, leaving a record of how someone arrived at a particular page.
All this information--IP address, date, time, user agent, HTTP cookie, and referring URL--gets recorded together. That makes it fairly easy to identify a Web user unless the person is sophisticated enough to use software to conceal or falsify the tracks he or she has left behind.
What Google Search Reveals About Us