Before we started to develop a silverlight application, does anyone ask the question? Is silverlight the windows or web?
The short answer is Windows, but wait....
All the code was executed on the client side, the code was download was downloaded to the client side.It has to be executed under the silverlight ActiveX control or browser plugin(For Firefox, Chrome, etc).
But why we still call the silverlight as a web application? Because it can interact with the HTML Dom, it can access the IIS by using the WCF, REST service. It's running under the browser. It's the light version of WPF and it's even called WPF/E!
So I will rather consider the silverlight as a web application , but I will be more concerned about the security while I am developing my application because the code was executed at the client side.