Greasemonkey is a popular browser extension that allows users to customize the way web pages look and function. It enables the installation of small scripts, known as user scripts, which can modify the content and behavior of websites. This customization can enhance user experience by adding new features, removing unwanted elements, or automating repetitive tasks. The extension is primarily used with the Mozilla Firefox browser, but similar functionality is available in other browsers through extensions like Tampermonkey. Users can easily find and install scripts from various online repositories, making it a versatile tool for web enthusiasts and developers alike.