GreaseMonkey is a popular browser extension that allows users to customize the way web pages look and function. It enables users to write and install small pieces of JavaScript code, known as "user scripts," which can modify the content and behavior of websites. This can enhance user experience by adding features, removing ads, or changing the layout of a page. The extension is primarily used with the Mozilla Firefox and Google Chrome browsers. Users can find and share scripts through various online repositories, making it easy to enhance their browsing experience based on personal preferences and needs.