One of the main objectives of ASP.NET MVC is Search Engine Optimization (SEO). Search Engines work mainly using URLs. Defining a meaningful and more understandable URL is very important to make our application more search engine friendly. Routers are used for defining a meaningful URL for the searchengines.
For more on Controllers and Routers, please refer my article http://www.codeproject.com/KB/aspnet/ASPNetMVCController.aspx