Extensions are additional features or tools that enhance the functionality of software applications, particularly web browsers like Google Chrome or Mozilla Firefox. They allow users to customize their browsing experience by adding new capabilities, such as ad-blocking, password management, or productivity tools. These extensions are typically available through online stores or repositories, where users can easily search for and install them. Once installed, extensions integrate seamlessly with the browser, providing quick access to their features. Users can manage these extensions through the browser's settings, enabling or disabling them as needed.