What are Trackers, Supercookies, and E-Tags?

With the advent of the Internet and its associated benefits, there has been a sharp incline in the collection of one's digital traces. Every time we surf the net, visit a particular website or make an online purchase, our activity is monitored, and the relevant data is stored.

Example

Suppose we are browsing a website because we are interested in purchasing a fishing boat. Then, we switch to another unrelated website about kitchen appliances, but there is an advertisement for the same fishing boat we were looking at earlier. This is not a coincidence—it is all made possible through web trackers.

Trackers

Web trackers are pieces of code (or scripts) on websites that gather data points about their users, such as their preferences and how they interact with the website. The primary purpose of tracking is to create an accurate user profile in order to advertise in a targeted manner.

There are two main classes of trackers:

First-party trackers: These are administered directly by the host website or any of its service providers. Examples of first-party tracking include remembering log-in credentials and storing the contents of shopping carts.
Third-party trackers: These are implemented by external organizations seeking to gain knowledge about our digital habits to advertise to us.

There are four main examples of trackers:

Cookies
Supercookies
E-Tags
Fingerprinting

This answer will focus on supercookies and E-Tags.

Supercookies

Supercookies are similar to traditional cookies because they allow web tracking and collection of user preferences. However, unlike regular cookies, they are permanently stored on the user's device and are independent of the browser. This makes their detection and deletion difficult.

E-Tags

E-Tags, also known as entity tags, are a form of cache-based tracking used to identify and validate resources visiting a website.

Note: To know more about caching, please refer to this answer.

How it works

E-Tags function in the following way:

Advertisers can embed $1 \times 1$ tracking pixels on a website. These are stored in the cache by the browser and assigned an E-Tag, an HTTP response header consisting of a unique key-value pair.
When the user re-visits the website, the associated elements are returned to the web server.
This facilitates re-identification of the user in case they delete the cookies or use private mode to access the website.

The mechanism is outlined below: