Originally silverlight was referred to as WPF/E. Silverlight enables development of rich interactive applications(RIAs) for the web. Silverlight is a free cross-platform and cross-browser plugin which can be downloaded by the user. Silverlight plug-in is a scaled-down version of the full .NET Framework. The Silverlight plug-in contains only those classes and functionality which are applicable to a Silverlight Web client. The classes and functionality that Silverlight plug-in provides are streamlined and optimized for use on the Web client machine. Silverlight comprises a subset version of the .NET Framework and WPF(Windows Presentation Foundation). Silverlight uses Extensible Application Markup Language (XAML) to construct the user interface (presentation layer) just like WPF in .NET 3.0. Silverlight applications are comprised of text-based files that include XAML and code. Silverlight applications can be created using any text editor. Silverlight supports managed code like VB,C#,Ruby,Python and also XAML extensibility and control class and sample controls.
.NET Framework is used by both ASP.NET and Silverlight for web development. However ASP.NET and Silverlight functioning varies with eachother. Most of Silverlight processing is carried out on the clientside as opposed to that of ASP.NET. This enables better web experience and decreases server resource utilization. Like ASP.NET Silverlight too has an associated code behind file that includes the code that handles events fired by the page.