Online profiling: By using cookies, sometimes combined with personal information, sites build profiles about their customers, collating their preferences and tastes.

Registration: Most sites usually force surfers to register with them before they can view the contents or buy something or sign up for a service. This data is then used to target advertisements or sold to other websites.

Ad Networks: They are the Web equivalent of ad agencies with the most successful being DoubleClick, Engage and 24/7 Media. They collect millions of profiles of Web surfers and use them to target personalised ads for their clients. For example, a surfer at Yahoo from India would be shown an ad from an Indian company, while at exactly the same time, an American surfing at Yahoo might be shown an American ad. Similarly, a person interested in finance may be shown ads related to finance, while an ardent reader who has bought several books on the Net may be targeted with ads on the latest books.

IP Address: A number automatically assigned to a computer whenever the computer connects to the Net. These numbers are used by network computers to identify the PC so that data can be transferred accurately. However, IP addresses, especially those that are not dynamically assigned ie., those that are fixed, can also be used for profiling and ad targeting.

Privacy policies: Notices posted on websites that disclose how a company collects, uses and shares data with partners or advertisers.

Opt-in and opt-out: Privacy choices that some websites offer to their visitors.In opt-out situations, the site is free to gather and sell information on you unless you specifically tell it not to.