ASP.NET Web Forms uses Page controller pattern approach for rendering layout. In this approach, every page has it's own controller i.e. code-behind file that processes the request. On the other hand, ASP.NET MVC uses Front Controller approach. In this approach a common controller for all pages, processes the requests.
ASP.NET Web Forms: ASP.NET Web Forms use Page controller pattern approach for rendering layout. In this approach, every page has its own controller, i.e., code-behind file that processes the request.
No separation of concerns. As we discussed that every page (.aspx) has its own controller (code behind i.e. aspx.cs/.vb file), so both are tightly coupled.
ASP.NET MVC: ASP.NET MVC uses Front Controller approach. That approach means a common controller for all pages processes the requests. Very clean separation of concerns. View and Controller are neatly separate. Testability is a key feature in ASP.NET MVC. Test driven development is quite simple using this approach.