The entry point of Silverlight applications is Silverlight Application class. It provides various services which is commonly needed by Silverlight application.
Silverlight lifecycle commences with opening a web page for hosting Silverlight plug-in. If the plug-in is not available, the web page prompts to install Silverlight browser plug-in. Later the browser activates the plug-in and starts downloading application package. This plug-in loads the Silverlight Common Language Runntime by which application domain for application is created.
Later, CLR creates an object of Application class followed by raising the Application Startup Event. There will be only one Application instance in any Silverlight-based application. Upon starting up the application instance provides several commonly used services by the application.