What is Silverlight?

Silverlight is a new cross-browser, cross-platform
implementation of the .NET Framework for building and delivering the next
generation of media experiences and Rich Interactive Applications(RIA) for the
web. It runs in all popular browsers, including Microsoft Internet Explorer,
Mozilla Firefox, Apple Safari, Opera. The plugin required to run Silverlight is
very small in size hence gets installed very quickly.

It is combination of different technolgoies into a single
development platform that allows you to select tools and the programming
language you want to use. Silverlight integrates seamlessly with your existing
Javascript and ASP.NET AJAX code to complement functionality which you have
already created.

Silverlight aims to compete with Adobe Flash and the
presentation components of Ajax. It also competes with Sun Microsystems' JavaFX,
which was launched a few days after Silverlight.

Currently there are 2 major versions of Silverlight:

Silverlight 1.0 and Silverlight 2.0( previously referred to
as version 1.1).

Main features of Silverlight 2.0 :

1. A built-in CLR engine that delivers a super high performance
   execution environment for the browser. Silverlight uses the same core CLR engine
   that we ship with the full .NET Framework.
   
2. Silverlight includes a rich framework library of built-in
   classes that you can use to develop browser-based applications.
   
3. Silverlight includes support for a WPF UI programming model.
   The Silverlight 1.1 Alpha enables you to program your UI with managed code/event
   handlers, and supports the ability to define and use encapsulated UI controls.
   
4. Silverlight provides a managed HTML DOM API that enables you
   to program the HTML of a browser using any .NET language.
   
5. Silverlight doesn't require ASP.NET to be used on the
   backend web-server (meaning you could use Silverlight with with PHP on Linux if
   you wanted to).
   
6. Silverlight 2 includes Deep Zoom, a technology derived from
   Microsoft Live Labs Seadragon. It allows users to zoom into, or out of, an image
   (or a collage of images), with smooth transitions, using the mouse wheel. The
   images can scale from 2 or 3 megapixels in resolution into the gigapixel range,
   but the user need not wait for it to be downloaded entirely; rather, Silverlight
   downloads only the parts in view, optimized for the zoom level being viewed.
   
7. Silverlight 2 also allows limited filesystem access to
   Silverlight applications. It can use the operating system's native file dialog
   box to browse to any file (to which the user has access).