What Is a Tracker? How Online Tracking Works and How to Stop It

A tracker records your behaviour and reports it to a third party. On a website, a script loads in the background and logs where you came from, what you click, how long you stay and where you go next. That data goes to an ad network or analytics provider, which combines it with what it saw on other sites.

The common types:

• Cookies: small text files your browser stores and sends back on later visits. A cookie set by an ad network that is embedded on many sites can recognise you on all of them.
• Tracking pixels: invisible 1x1 images in emails and web pages. Loading the image tells a server that you opened the mail or visited the page.
• Browser fingerprinting: a profile built from your screen size, fonts, browser version and dozens of other properties. It needs no cookies at all.
• SDKs in apps: analytics and advertising code that developers build into their apps. It reports straight from your device which features you use, how often and for how long.

The last type matters most on a Mac, because desktop apps reach their analytics services without any browser involved. NetMute lists the domains each app on your Mac contacts and marks the known tracking domains among them, so you can see which apps report on you. Looking is free.

If you use Google's services while signed in, Google can store your searches, your location history (if it is turned on), your YouTube watch history, Gmail, calendar and contacts. On an Android phone it also learns which apps you open and when. You can review and delete most of it at myactivity.google.com.

Meta reaches far beyond its own apps. The Meta Pixel is built into a very large number of websites and reports visits and purchases back to Meta. That is cross-site tracking: you look at shoes in an online shop, and later the same shoes appear as an ad on Instagram, because the pixel reported your visit.

Data brokers add another layer. They buy and combine data from many sources, and some advertise profiles with thousands of attributes per person: estimated income, interests, household, sometimes health or political leanings. These profiles are sold for advertising and for other purposes.

A browser ad blocker covers only part of this. It sees what happens inside the browser, not what the other apps on your Mac send. NetMute's Tracker Shield blocks known tracking domains for your apps as well as for the browser. In browsers that resolve names on their own, such as Brave, name-based blocking needs one extra setting; the app's "Where NetMute protects" check shows where blocking works.

"I talked about running shoes yesterday, and today I see ads for them." There is no public evidence that phones routinely record conversations to target ads. Tracking does not need a microphone: your location, searches and purchases, plus what people near you search for, are often enough for a well-aimed ad. If a friend searched for running shoes while you were both on the same Wi-Fi, an ad network can link the two of you through the shared IP address.

Smart TVs are a documented case. Many use automatic content recognition (ACR), which identifies what is on the screen, whether streaming, broadcast TV or a connected console. In 2017 Vizio settled with the US Federal Trade Commission over collecting viewing data from millions of TVs without consent. ACR can usually be switched off in the TV's privacy settings, often under a name like "viewing information".

Other connected devices send data too: smart speakers, robot vacuums, fitness trackers, connected light bulbs. On your Mac you can at least decide what your own computer sends. NetMute's Tracker Shield blocks 300+ known tracking domains (advertising, analytics, social networks, fingerprinting, crash reporting and telemetry) and shows which app tried to reach each one.

You can limit what Meta links to your account. In the Accounts Center, under Your information and permissions > Your activity off Meta technologies, you see which sites and apps sent data about you, and you can disconnect past and future activity. Sites still send that data to Meta; it is just no longer tied to your profile. The pixels on those sites keep running.

For Google, My Ad Center (myadcenter.google.com) lets you switch off personalised ads. On Apple devices, App Tracking Transparency makes apps ask before they track you across other companies' apps and websites. In your browser you can block third-party cookies in the privacy settings; Safari and Firefox already do it by default.

Do all of these. They share one weakness: each covers one service or one browser. Fingerprinting needs no cookies. SDKs in desktop apps ignore every browser setting. Some trackers hide behind a subdomain of the site you are visiting (CNAME cloaking), so they look like the site's own traffic.

To close those gaps you need a layer that works on network connections rather than on the settings of one service.

A tracker that cannot reach its server cannot send anything. That is the idea behind blocking at the network level. A browser extension only sees the browser. Your mail client, music app, messenger and the utilities in your menu bar make their own connections, and no browser plugin reaches them.

NetMute works at this level on a Mac. It shows in real time which app connects where. Tracker Shield blocks known tracking domains without asking you about each connection: the free version covers the 50 most active ones, Pro the full list of 300+. If blocking a domain breaks something in one app, you can allow it for that app alone.

The difference from a DNS filter or a VPN with a blocklist: NetMute ties every connection to the app that made it. Instead of an anonymous list of blocked domains, you see that, for example, a music app tried to reach an advertising network and was stopped. App X-Ray shows each app's contacted domains and a tracking rating for free; blocking straight from App X-Ray is part of Pro.

NetMute is free on the Mac App Store and needs macOS 26 or later. Pro is a one-time purchase with no subscription and no account, and nothing leaves your Mac by default.