What Is a Browser Extension? How Extensions Work, Explained Simply

What is a browser extension, in plain terms?

A browser extension is a small program you add to Chrome (or another browser) to give it one new ability the browser doesn't have by default. It installs directly into the browser, not onto your computer as a separate application.

Think of your browser as a phone. The browser itself only does browsing — loading pages, managing tabs, handling bookmarks. An extension is like an app you add to give it one specific new job: blocking ads, checking your grammar, or timing how long you spend on a site.

• Blocking ads and trackers before they load
• Translating a page's text into another language
• Reading text aloud with text-to-speech
• Saving passwords and filling them in automatically
• Adding a timer or dashboard on top of sites you already use
• Capturing a screenshot or clipping a page into another app

What can a browser extension actually do?

Extensions can read and change what's on a page, run in the background, add buttons to your toolbar, and store settings — but only within limits the browser and the extension's own declared permissions set.

How does an extension get onto a web page? (Content scripts)

Most extensions use a content script — a small piece of code the browser injects into the page you're viewing, after that page has already loaded.

A content script can read the page's text, insert new elements like buttons or overlays, and listen for things like clicks or text selections. For security reasons, it runs in an isolated context, separate from the page's own JavaScript, so a content script and a website's code can't directly interfere with each other.

A highlighter extension like Weava Highlighter or Web Highlights is a clear example: it injects a small toolbar into the page and lets you mark up text, even though the website itself never built that feature. The extension only runs on pages that match the rules it declared when it was installed, not on every site you visit.

What is a background script, and why does an extension need one?

A background script (often called a service worker in current Chrome extensions) is the part of an extension that runs independently of any single open tab.

It handles the tasks a content script can't, because a content script only exists while its specific page is open. A tool that tracks Canvas assignments in the background, like Enhanced Canvas LMS To Do, needs something running even when you're not on the Canvas tab, to check for new work and update a badge count.

• Checking for updates or new data on a timer
• Updating a badge count or icon before you open the relevant tab
• Relaying messages between an extension's popup and its content script
• Keeping a setting or a small amount of data synced across tabs and browser sessions

Why do browser extensions ask for permissions, and what do they mean?

Permissions tell you, before you install, exactly what an extension is allowed to see or do. Chrome shows these on the extension's Chrome Web Store listing and again in the install prompt.

Read and change data on this site

The extension can see the page's content and modify what's shown, which is how translators, highlighters, and readers work.

Read your browsing history

Needed by tools that act on which sites you've visited, such as time trackers or history-based recommendations.

Manage your downloads

Lets an extension save files directly, used by clippers and screenshot tools that export what you capture.

Display notifications

Allows the extension to show a system notification, separate from anything happening inside a tab.

How is a browser extension different from an app or a plug-in?

A web app runs at a URL with no install step, a native app installs on your operating system and works without a browser open, an old-style plug-in ran separate software inside a page, and an extension installs into the browser itself and only runs while that browser is open.

• Web app: no install, works at a URL, needs an internet connection and a browser tab to run.
• Native app: installed on the operating system, runs independently of any browser.
• Plug-in: an older concept (Flash, Java applets are the well-known examples) that ran separate embedded software inside a page; modern browsers, including Chrome, no longer support this model.
• Extension: installed into the browser through its official store, only active while that specific browser is running.

This is also why an extension you install in Chrome doesn't show up in Firefox or Safari, and why closing your browser fully stops every extension's activity along with it.

How do you know if a browser extension is safe to install?

Check whether the permissions it asks for match its stated job, who publishes it, and whether it's actively maintained, before you install anything.

• Compare the permissions requested against what the extension claims to do — a bookmark manager doesn't need microphone access.
• Look for a real, identifiable developer with a support page or contact method, rather than an anonymous listing.
• Check when it was last updated; an extension untouched for years is more likely to have unpatched issues.
• Only install from the official Chrome Web Store, not from a link on a random website promising a 'faster' version.
• Read the extension's own privacy disclosure on its Chrome Web Store listing about how it uses your data.

For a closer look at how we evaluate extensions before recommending them, see our methodology.

How do you install, disable, or remove an extension in Chrome?

All three actions happen from the same two places: the Chrome Web Store to install, and chrome://extensions to manage.

1. Go to the Chrome Web Store and find the extension you want.
2. Click Add to Chrome.
3. Click Add extension in the confirmation pop-up.
4. Click the puzzle-piece icon in the toolbar to see and pin any installed extension.
5. To disable or remove one later, right-click its icon (or open chrome://extensions), then toggle it off or click Remove.

Why would you add a browser extension instead of just using a website?

You add an extension when the feature needs to work across every site you visit, not just one — a dictionary lookup, a translation overlay, or a research clipper only make sense if they follow you from page to page.

For a broader list of terms like content script and permission, browse our glossary, and for extensions grouped by what they're actually used for, see our collection for remote work.