Silverlight is a browser plug-in developed for delivering rich internet applications (RIAs) across multiple platforms and devices, originally known as WPF/E. It allows for advanced features such as video playback, 3D graphics, and seamless integration with existing web technologies while supporting various programming languages. Despite its rich capabilities, Silverlight has struggled to penetrate the market dominated by Adobe Flash, primarily due to its browser-only solution and limited development support on Windows.