Many organizations use ``cookies'' to track your every move on their site. A cookie is a unique identifier that a web server places on your computer: a serial number for you personally that can be used to retrieve your records from their databases. It's usually a string of random-looking letters long enough to be unique. They are kept in a file called cookies or cookies.txt or MagicCookie in your browser directory/folder. They are also known as ``persistent cookies'' because they may last for years, even if you change ISP or upgrade your browser.
If you look at your cookies file you may see the names of web sites that you have never heard of. They were probably put there by companies that resell advertising space from a large number of popular sites. Those ad placement companies maintain huge databases recording details of who looks at which pages. The larger ones have cookies in place on millions of peoples' browsers.
If you use one of the popular search engines, the queries you type are probably being logged and analyzed too. We wonder whether some companies are selling your identity as part of the package.
Any web site that knows your identity and has cookie for you could set up procedures to exchange their data with the companies that buy advertising space from them, synchronizing the cookies they both have on your computer. This possibility means that once your identity becomes known to a single company listed in your cookies file, any of the others might know who you are every time you visit their sites.
The result is that a web site about gardening that you never told your name could sell not only your name to mail-order companies, but also the fact that you spent a lot of time one...