A CRX file is a package file format used by Chromium-based browsers like Google Chrome, Microsoft Edge, and others to distribute extensions, themes, and apps. It's essentially a ZIP archive containing the code, resources (images, scripts, HTML, CSS), and a manifest file (manifest.json) that describes the extension's metadata, permissions, and functionality. When you install an extension from the Chrome Web Store or load a CRX file directly, the browser unpacks the archive and registers the extension, allowing it to modify the browser's behavior, add new features, or integrate with web pages. CRX files are digitally signed to ensure their authenticity and prevent tampering. The signature verifies that the extension hasn't been modified since it was created by the developer. This helps protect users from malicious extensions. The manifest file is crucial as it declares the permissions the extension requires, such as accessing browsing history, tabs, or web content. Users should carefully review these permissions before installing an extension to understand its potential impact on their privacy and security.