Most internet users are probably aware that the websites they visit may collect information on them. It is common practice for a website to record details such as how often a user visits the website, and even how a user spends time on the site -- tracking, for example, what articles a user reads or what books a user buys.
However, many users may not be aware that major advertising companies are aggregating this information together from many different websites. If you use your browser's default privacy and security settings, you are being tracked from site to site. With each additional site you visit, you give away another piece of information on who you are.