It's similar to what Adobe Flash does. Microsoft Silverlight is for creating and running rich Internet applications. It's a cross-browser, cross-platform implementation of .NET. You don't really need any tools. With almost any text editor you can build these applications. If you want to get more productivity there are a few tools that you can download, such as Silverlight Toolkit that provides more components for developers to use. If you're a regular user and are not interested in developing anything using Microsoft Silverlight, you will probably just be asked to install this plugin for your browser in order for these applications to work.