Who is watching you on the web?

You’re never alone when you browse the web. We reveal who might be watching you when you visit your favourite websites.

ADVERTISERS

Online advertisers can monitor how well their campaigns are doing by studying how you browse the web. Typically, this is done using cookies - small pieces of data stored in your browser that let websites remember you and your personal preferences (although they don't identify personal details).

For example, the digital-marketing firm DoubleClick, which is owned by Google, uses a cookie that stores information about the ad campaign, your local time, your IP (Internet Protocol) address (a number allocated by your ISP when you go online), your operating system and web browser, the page the ad appeared on, whether you clicked on the ad, and the number of times you saw it.

The DoubleClick cookie is a ‘persistent' type, which means it stays in your browser until you delete it. Advertisers can then build up a complete picture of their campaigns over time, across the multiple websites their adverts appear on. You can permanently avoid the DoubleClick cookie by installing the Google Advertising Cookie Opt-Out plug-in, although this won't stop adverts from being displayed.

Companies can also track what you're doing online by combining cookies with web bugs, which are small images or pieces of code embedded in sites. In 2007, Facebook was forced to re-examine a controversial service called Beacon, which used web bugs to track what users were doing on the social-networking site, such as buying concert tickets or reviewing a book.

But Facebook failed to ask its users for explicit permission to publish their activities on friends' news feeds. Beacon still exists, but it now requires you to opt into the scheme, and you'll need to ‘OK' stories first.