If you look at a product on the web you may find that it follows you around, popping up in adverts on seemingly unrelated websites. This is one of the simplest forms of targeted advertising. When you visit a website, third-party advertisers leave tracking cookies in your browser, enabling them to monitor your online activity and remind you later of items or services you looked at earlier.
Search engines like Google collect data about the searches performed from a particular IP address, taking into account search terms, but also the user’s location. They even target advertising based on keywords collected from the messages in your email inbox. Your activity on Google-owned sites like YouTube is also fed back to advertisers for use in targeting.
Social networks sell data to advertisers too. Facebook uses the personal information that you provide about your life and interests, along with your ‘Likes’ and friendships, to help advertisers pitch to a suitable audience.