This ASP.NET web forms file contains the markup (or user interface code) of the file and optionally the underlying application code.
.cs or .vb files
These are the code-behind files. If the page has indicated so, the underlying application code will be created here. This is the default setting.
This is the applicationâ€™s general configuration file. It is an XML-based file that contains all settings for customizing the connection strings, application settings, security, external assemblies, memory, state management, and so on.
In this file, you can add code for event handlers at the application level. Events are those for when the application starts or ends or when an error is thrown.
These are user control files. In these files, you can create small pieces of functionality the same way as with a full ASPX page, but the difference is that they can not be accessed directly and must be hosted inside ASPX pages. You can reuse these user controls in any page of your application.
.asmx or .svc files
ASMX files are ASP.NET web services. These files provide services for pages in your application or any other program that can access them. ASMX web services are
being slowly replaced by Windows Communication Foundation (WCF) services, which have the extension .svc and offer improved security and scalability features.
Master pages are like ASPX pages with the difference that they are used as templates for other pages, sharing the look and feel and base functionality. Master pages inside other master pages are called nested master pages.