If you followed the Facebook hearings, there’s one term that came up many times: “cross-site tracking” — and for good reason. But it also seems to be something that many people do not understand. So what does cross-site tracking even mean and why is it important right now? Read on.

What it is

This is the easiest part! It’s exactly what it sounds like: cross-site tracking generally refers to companies collecting browsing data across multiple websites.

When you browse from site to site, you’re often followed by trackers that collect data on where you’ve been and what you’ve done, using scripts, widgets or even tiny, invisible images embedded on the sites you visit. Take, for example, those social share buttons embedded on many websites. Sites may choose to include those buttons to gain useful analytics about their content, but the buttons also send data back to the social platforms. Sometimes, that makes sense, allowing you to share content on other social platforms. But often, that data also ends up being used behind the scenes to target advertising or create user profiles.

What also happens behind the scenes is that many more third parties — companies separate from the sites you’re visiting — are also receiving that activity, without your knowledge.

Why it exists

Most sites collect data to personalize your experiences with them and make their site work better. That’s pretty widely accepted as normal. It usually does make browsing easier and the publisher’s services better. They often use cookies — small data files stored by your browser — to remember things like language preferences or what you’ve got in your shopping cart.

Cookies can also be used to follow you around the web and serve up ads based on the things you’ve looked at. Here’s how it works: You visit a site, a third-party advertiser leaves a cookie on your browser. The cookie may contain a unique identifier — not necessarily your name, but something they can use to understand who you are and which sites you visit. Details about what you do from that point on can be transferred to a third party and stored on remote servers. Those cookies help advertisers serve you custom ads across the web and in your social media feeds, and can also reveal what you’ve been doing online to a social site, which is why some ads will follow you into social media walled gardens.

For many, having ads that are in tune with what you’re searching for is a plus. If you’re into ceramic pottery and succulents, it’d be annoying and weird to see ads for medical equipment or industrial pipe and valve fittings (been there, seen that).

Regardless, the most important part is that you should still have control over what advertisers know about you, if they know anything about you at all, which can be tough when web trackers operate in the background – out of sight.

When it’s potentially problematic

Not knowing what’s up with your data is what makes cross-site tracking tricky. Those third parties — like data brokers, affiliate networks and advertising networks — use cookies, and other data tracking methods, to collect information about our browsing habits without our consent.

While tracking can be helpful, there can’t be a healthy trade-off between the value it provides and the data it collects without transparency (aka being informed, in a way that’s easy to understand, who sees your data, who it’s shared with and you having the option to say “Um, no. I’m out.” if you don’t like it).

How you can take control

One way to dial down the creepiness factor is to use Private Browsing with Tracking Protection right in Firefox, which makes it harder for third-parties to track your search history across multiple sites. It’s available on our desktop and mobile browsers (cause tracking happens regardless of the device you connect with).

When you browse in a Private Window, Firefox doesn’t save the pages you visit, cookies, searches or temporary files. It’s not a bulletproof solution, but for everyday web browsing it might just give you some peace of mind.

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