Silverlight is a Microsoft web application framework for building and delivering rich Internet applications and multimedia. It allows developers to build cross-browser, cross-platform desktop and mobile applications using HTML, CSS, and JavaScript. Silverlight applications can be developed with C# or VB.NET using Visual Studio and the Model-View-ViewModel (MVVM) pattern. It supports out-of-browser and Windows Phone applications and integrates with other Microsoft technologies like .NET, SQL Server, and Azure.