What controllers are and how to create them, and how ASP.NET invokes a controller via URLs ("routing").
This MSDN article explains what controllers and action methods are.
This Pluralsight video explains controllers and routing.
Stephen Walther shows how the ASP.NET MVC framework maps browser requests to controller actions.
ASP.NET routing enables you to use URLs that do not have to map to specific files in a Web site. Because the URL does not have to map to a file, you can use URLs that are descriptive of the user's action and therefore are more easily understood by users.
Scott Guthrie drills deeper into the routing architecture of the ASP.NET MVC Framework, and discusses some of the cool ways you can use it for more advanced scenarios in your application.
This MSDN article explains filters, which let you run code either before an action method is called or after an action method has run.
See "Understanding controllers" in the ASP.NET MVC Content Map