[Sir_Cmpwn]

There are a number of ways. Cookies are one, but you can also collect other kinds of data from a web browser to uniquely identify a user across multiple sessions. Generally speaking, if you can run JavaScript, you can track the user. This is done by all advertisers and most little widgets like Facebook or Disqus comments, like and tweet buttons, etc.

[amelius]

> This is done by all advertisers and most little widgets like Facebook or Disqus comments, like and tweet buttons, etc.

Why isn't this illegal already?

[notalaser]

Because it's highly lucrative.

[oneplane]

And because it's very functional and easy at the same time.

[JumpCrisscross]

Most people don't know nor care about the issue.