Microsoft Silverlight : An Introduction

WHY ?

1. It supports playback of WMV files on both PC and Macintosh, with many options for interactivity during playback; with just a couple of lines of code, you can provide a platform-neutral way to handle all your movie files. Silverlight supports full-screen 720p video and offers seamless transitions between full-screen and windowed mode without losing your position in the video (something that media sites are crying out for today).
2. By separating markup (XAML) from code, Silverlight provides a familiar web metaphor for designers and developers. You can embed XAML directly within an HTML file if you want a simple, monolithic solution, or you can keep the two separate to enforce a delineation between different web development roles.
3. Silverlight and HTML integrate seamlessly together. Every XAML element can be accessed or manipulated from the same client-side JavaScript that would be used to interact with any DHTML element: there are no artificial boundaries or barriers, and you can even overlay HTML elements on top of Silverlight content (simply by creating a windowless frame). We'll also make it very easy for an ASP.NET AJAX developer to add Silverlight content.
4. You can embed XAML directly into your HTML pages; there's nothing binary or opaque about the format. There are only three steps necessary to add animation or media to your RIA application: (i) include a standard JavaScript file in your HTML header; (ii) call a function to create the Silverlight object anywhere on the screen; (iii) add some XAML content (an animation, some media) for runtime delivery.
5. You have full runtime interactivity with Silverlight content. The contents of the XAML file can be completely server-generated, to contain information populated from a database. From JavaScript, it's just a matter of calling the createFromXaml method to add or remove elements dynamically at runtime. There's nothing that you can only create or manipulate at design-time.
6. Silverlight is just a 1MB download on a PC (slightly more on a Macintosh because the universal package contains both Intel and PowerPC versions); it supports Windows XP and above, with Windows 2000 support to come.
7. Silverlight is blindingly fast - for example, you can play many videos simultaneously without stuttering or dropping frames (subject to network bandwidth, of course). We're introducing a new video brush in Silverlight that allows you to use video as a texture for any 2D object (a rectangle, an ellipse or a path). This is going to allow designers incredible power to use media in new ways that have never been accessible through other existing technologies.
8. Silverlight is both client- and server-agnostic. There's no difference between the Macintosh and PC runtimes; you don't need any Microsoft software on the server if you don't want to - you can deliver a great Silverlight experience from an Apache / Linux server to a Mac OS 10.4 client.
9. Silverlight is almost 100% upward compatible with WPF. Animation, 2D vector graphics, media, text - they're all present in Silverlight and the concepts you've learnt in WPF carry forward (although Silverlight is a subset - it doesn't support WPF features such as 3D, data binding or templates). You can use the same tools (e.g. Expression Design) to generate content for Silverlight; you can take XAML from Silverlight and use it in a WPF application when you want to scale up and take full advantage of your local machine.

Src: http://blogs.msdn.com/tims/archive/2007/04/15/introducing-microsoft-silverlight.aspx1