ASP vs ASPX:
Think of ASPX files as the next generation of ASP (active server pages) that greatly extend the capabilities of regular HTML web pages. Also known as ASP.NET, it is a programming framework built on the common language runtime that can be used on an ASPX hosting server to build powerful Web applications. ASP.NET and ASPX files offer several important advantages over previous Web development models:

Enhanced Performance: ASPX is compiled common language runtime code running on the server. Unlike its interpreted predecessors, ASP.NET can take advantage of early binding, just-in-time compilation, native optimization, and caching services right out of the box. This amounts to dramatically better performance before you ever write a line of code.

Scalability and Availability: It has been designed with scalability in mind, with features specifically tailored to improve performance in clustered and multiprocessor environments. Further, processes are closely monitored and managed by the ASP.NET runtime, so that if one misbehaves (leaks, deadlocks), a new process can be created in its place, which helps keep your application constantly available to handle requests.

World-Class Tool Support: The ASPX framework is complemented by a rich toolbox and designer in the Visual Studio integrated development environment. WYSIWYG editing, drag-and-drop server controls, and automatic deployment are just a few of the features this powerful tool provides.

Power and Flexibility: Because ASPX is based on the common language runtime, the power and flexibility of that entire platform is available to Web application developers. The .NET Framework class library, Messaging, and Data Access solutions are all seamlessly accessible from the Web. 

ASP.NET is also language-independent, so you can choose the language that best applies to your application or partition your application across many languages. Further, common language runtime interoperability guarantees that your existing investment in COM-based development is preserved when migrating to it.

Simplicity: It makes it easy to perform common tasks, from simple form submission and client authentication to deployment and site configuration. For example, the page framework allows you to build user interfaces that cleanly separate application logic from presentation code and to handle events in a simple, Visual Basic - like forms processing model. Additionally, the common language runtime simplifies development, with managed code services such as automatic reference counting and garbage collection.

Manageability: It employs a text-based, hierarchical configuration system, which simplifies applying settings to your server environment and Web applications. Because configuration information is stored as plain text, new settings may be applied without the aid of local administration tools. This "zero local administration" philosophy extends to deploying ASPX Framework applications as well. A Framework application is deployed to a server simply by copying the necessary files to the server. No server restart is required, even to deploy or replace running compiled code.

Customization and Extensibility: It delivers a well-factored architecture that allows developers to "plug-in" their code at the appropriate level. In fact, it is possible to extend or replace any subcomponent of the runtime with your own custom-written component. Implementing custom authentication or state services has never been easier.

Security: With built in Windows authentication and per-application configuration, you can be assured that your applications are secure.