A tracker is a company which collects information about you as you browse the web. Their scripts and tracking pixels are inserted into the websites you visit by publishers or advertising networks, usually in exchange for some service that the tracking company provides.
Suggested reading: If you want to find out more about internet trackers, check out our blog - What is tracking and how can I browse safely?
Trackers use identifier tools to link information about you from different sites. This data is used to build up a user profile, based on your browsing history.
Tracker profiling places users in groups and sells data to third parties so they can target certain users online.
We define a tracker as a third-party domain which is:
> 10) different websites with a significant combined traffic
The majority of the companies behind trackers offer services to website publishers. The publishers will put scripts or tracking pixels for these services onto their sites.
There are many different use-cases for these services, such as:
Many of these use cases do not require tracking. However, they may track your behavior by accident because the implementation is easier, or on purpose because the data generated can be used to increase revenue.
TIP: Want to find out who’s tracking you online? Check out Ghostery’s Trackers Preview Wheel.
Trackers use user identifiers to group data received from each unique user, and browsers send the URL of the first-party page to third-party resources in the page as standard.
This means trackers are effectively able to collect a subset of users' browsing history. Even if this history is linked to an 'anonymous' identifier, any browsing history of reasonable length can be deanonymised to find the user behind it.
In the majority of cases, trackers are invisible on the pages they appear. This means there is little or no transparency about who is logging activity on a site, hence the need for privacy tools such as Ghostery to call them out.