Let's say you're going out on your very first date at the local mall. Your parents, although very supportive, worry about you... a lot. They want to know who you're with, where you're going, what you're doing, and when you'll be back. And they want this information on demand. However, instead of asking you to send them a periodic SMS report on your whereabouts...
They paid the mall staff to do the reporting.
So if you went to see a movie, the theater staff would report the movies you watched and the refreshments you purchased. If you went to eat, the restaurant staff would report who you're with and the food you ordered. If you went to play games, the arcade staff would report the games you played and the prizes you won. If you moved from place to place, the security staff would report where you are and what you did along the way. And the list goes on.
Web tracking works in a very similar manner. The parents would be the tracking service. The mall venues would be the websites, with the mall staff being the embedded tracking scripts. And of course there's you. Tracking scripts report to their servers your movement across websites in the same way the mall personnel in the analogy are reporting to your parents your movements within the mall.
And oh, did I say paid? Yes, I did. Most web trackers offer incentives to website owners in exchange for taking part in tracking. Just like how the parents in the analogy paid a lot of mall staff to cover every corner of the mall, web trackers also give out compensation or offer a free service just to get their tracking scripts on as many websites as possible. More ground covered equates to greater visibility on user movement.
Web tracking is not rocket science. It's like connect-the-dots drawing. The dots are not interesting on their own. But if you connect the dots, you could end up with a pretty drawing. And to get a bigger, more intricate, and possibly more valuable drawing, you have to work with more dots.
Hopefully this article gave you insight on how you're being followed online. As always, if you have comments or suggestions, feel free to drop a line.