Fancybox is a popular JavaScript library used to create responsive lightbox-style overlays for displaying images, videos, and other content on websites. It allows users to view media in a larger format without leaving the current page, enhancing the overall user experience. The library is easy to implement and can be customized to fit the design of a website. Developed by Fancybox creator Fancybox, the tool supports various media types, including HTML, iFrames, and AJAX content. It is widely used by web developers and designers to showcase galleries, portfolios, and multimedia presentations in an elegant and user-friendly manner.