ASP.NET MVC is driven by the URLs your users provide to get to the Views they want to see. If those URLs don't include a controller or action method name, then you can provide the missing information ...
When a request comes in to your ASP.NET site, it's routed through a series of message handlers (in ASP.NET Web API) or modules (in ASP.NET MVC), each of which performs some operation on the request.