What is SilverLight and can be useful in C# Application?
A simple look at Wikipedia would give you all the answers: http://en.wikipedia.org/wiki/Silverlight.
In short: "Microsoft's answer to Flash", if you will.
You can read http://en.wikipedia.org/wiki/Silverlight but quick answer is:
Is the "Flash" equivalent technology of Microsoft. It can be used in web sites to have richer content than HTML+ JavaScript. But recent version of HTML (HTML5) threatens Flash and SilverLight...
The main disadvantages of SilverLight and Flash technologies are that you need a "plugin" installed in all the machines that are going to visit the site. With HTML + JavaScript you only need the browser.
It is a Rich Internet Application (RIA) environment, similar to flash.
It uses XAML as a markup language and a subset of the .NET framework - so it can in the browser.
Wikipedia has more information, as do the official site and MSDN.
Silverlight is a powerful development platform for creating engaging, interactive user experiences for Web, desktop, and mobile applications when online or offline.
Refer to http://www.silverlight.net/.