There you are, wandering the web. You read some articles, watch some cat videos. All in a day’s work, right?

Then you notice something. Ads. Lots of ads.

It isn’t just the number of ads, though. There is something else going on.

You sent your friend an email about your surfing failures, and get ads for surfboards. You bought some shoes, and now get ads for the exact same pair on other websites.

What is going on? Is the government tracking you?

Yes. Though that has nothing to do with the ads.

Still, you aren’t paranoid. Well, you probably are, but you are right in this case: ads do follow you around the web.

But why?

Well, as it turns out, people like money. Ads make people money. Ads that show you what you want to buy make them more money.

If you are a surfer, you are probably more interested in buying surfboards than the average person. Similarly, if you bought shoes, you belong in the demographic that buys shoes. So you get ads… for shoes.

So why ads follow you is clear.

But how?

The primary tracking mechanism used by ads is cookies. Unfortunately, there are no chocolate chips in these cookies. At least you don’t have to worry about raisins.

Cookies in the context of the web are tiny files stored on your computer by websites. For example, let’s take Gmail. When you log into Gmail, you can check a box that says “stay logged in.” What it does is store a cookie. When you exit your browser and revisit later, your browser sends Gmail the cookie. The cookie contains information like your username. Gmail can then log you back in using this information.

Ads use cookies to identify you. Let’s imagine the information contained in the cookie is an id number. You can be internet person #135304449.

When you visit a website with an ad, information is learned about you. Let’s say you are perusing a surfing website. The ad there notes that internet person #135304449 likes surfing. This information is stored for later.

You also visit a fishing website. The ad there notes internet person #135304449 likes fishing.

And so on.

The next day, you visit a website about authentic range-grown magnetized mustard, being a connoisseur of extravagant mustards. There you find an ad for surfboards.

Huh? What do surfboards have to do with mustard? Nothing. At least, not that I know of. What you do with your mustard is your own business.

The ad knows from before that you like surfing. That part makes sense. But why does an ad on a different website remember who you are?

Ad Networks

The ads on all these websites are actually one and the same. Not the content of the ad – shoes, surfboards, and so on – but the tracking part.

The way most websites include advertising is by using ad networks. Ad networks are middlemen. Advertisers place ads through the ad network, and the ad network pays the website owner to put ads on their website. Everybody makes money and they all dance in a circle singing, because that is what you do when you are happy.

The ad network gives the website owner some code to include ads on the website. This code is what does the tracking.

The result is that the same tracking mechanism is spread across many different websites. Because all this information goes to one place – the ad network – all the ads from an ad network share the same information. The ad you see on a particular page might not have anything to do with that specific website. Instead, that particular ad could show anything based on the information already known about you.

This is why ads seem to follow you.

There is one more question that remains, though. At the beginning of this article, I mentioned that you sent an email to your friend about surfing. You didn’t go to a surfing website, or buy a surfboard. So why did you still get ads for them?

There is an explanation for this, at least if you use Gmail. This shouldn’t be news to you, but Gmail is from Google. As it so happens, the largest ad network, Adsense, is also owned by Google.

Or, in other words, the company that collects information about you to show you advertisements can also read your email. Conveniently, Google also has access to what you search for. Still got that tinfoil hat?

Gmail (a computer, not a person) scans your emails for a number of reasons (malware, spam, etc). One of these reasons is to collect information for advertising. For example, it finds the word surfboard in your email, and decides that you like surfing. This is why ads can be related to your emails.