Microsoft Silverlight is a technology for running applets within a .NET virtual machine embedded in the client browser.
Microsoft Silverlight can be viewed as the translation to .NET of the Java Applet model. Application code is pushed from a Web server to a client browser, within the context of a Web page; that code is executed in a sandbox which leverages the unescapable strong types of the Common Language Runtime (the VM which powers .NET) to allow for running the potentially hostile code without surrendering the browser's security to the whims of the contacted Web server.