Principles of ASP.NET
Even though ASP.NET takes its name from Microsoft's old web development technology, ASP, the two differ significantly. Microsoft has completely rebuilt ASP.NET, based on the Common Language Runtime (CLR) shared by all Microsoft .NET applications. Programmers can write ASP.NET code using any of the different programming languages supported by the .NET Framework, usually C#, Visual Basic.NET, or JScript .NET, but also including open-source languages such as Perl and Python. ASP.NET has performance benefits over other script-based technologies because the server-side code is compiled to one or a few DLL files on a web server.

ASP.NET attempts to simplify developers' transition from Windows application development to web development by offering the ability to build pages composed of controls similar to a Windows user interface. A web control, such as a button or label, functions in very much the same way as its Windows counterpart: code can assign it properties and respond to its events. Controls know how to render themselves: whereas Windows controls draw themselves to the screen, web controls produce segments of HTML and JavaScript which form part of the resulting page sent to the end-user's browser.

ASP.NET encourages the programmer to develop applications using an event-driven GUI paradigm, rather than in conventional web-scripting environments like ASP and PHP. The framework attempts to combine existing technologies such as JavaScript with internal components like "Viewstate" to bring persistent (inter-request) state to the inherently stateless web environment.

ASP.NET uses the .NET Framework as an infrastructure. The .NET Framework offers a managed runtime environment (like Java), providing a virtual machine with JIT and a class library.

The numerous .NET controls, classes and tools can cut down on development time by providing a rich set of features for common programming tasks. Data access provides one example, and comes tightly coupled with ASP.NET. A developer can make a page to display a list of records in a database, for example, significantly more readily using ASP.NET than with traditional web technologies like ASP or PHP.

ASPX file format
ASPX is a text file format used as web-form pages in the .NET environment. In programming jargon, the ASPX file typically contains only static HTML or XHTML code where the developer places all the required form fields and text content for the web page. The dynamic code that involves request and response from the server is placed in an HTML page with a tag or block <% -- dynamic code -- %> which is similar to other web development technologies such as PHP and JSP. ASP.NET does support inline code blocks inside an ASPX file, but this practice is generally discouraged. The ASPX and other resource files are placed in a virtual host on an Internet Information Services or other compatible ASP.NET servers. When a client requests information, the .NET framework parses and compiles the file into a .NET class and sends the response. Unlike some other web development technologies, which compile their files each time when responding the client, the ASPX files are compiled only for the first time when accessed and are then re-used to reduce the response time. Developers can also choose to pre-compile their code before deployment, eliminating the need for just-in-time compilation in a live environment.

Advantages of ASP.NET over ASP
Compiled code means applications run faster with more design-time errors trapped at the development stage.
Significantly improved run-time error handling, making use of exceptions and try-catch blocks.
User-defined controls allow commonly used templates, such as menus.
Similar metaphors to Windows applications such as controls and events, which make development of rich user interfaces, previously only found on the desktop, possible.
An extensive set of controls and class libraries allows the rapid building of applications.
ASP.NET leverages the multi-language capabilities of the .NET CLR, allowing web pages to be coded in VB.NET, C#, J#, etc.
Ability to cache the whole page or just parts of it to improve performance.
Ability to use the code-behind development model to separate business logic from presentation.
If an ASP.NET application leaks memory, the ASP.NET runtime unloads the AppDomain hosting the erring application and reloads the application in a new AppDomain.
Session state in ASP.NET can be saved in a SQL Server database or in a separate process running on the same machine as the web server or on a different machine. That way session values are not lost when the web server is reset or the ASP.NET worker process is recycled.
Previous versions of ASP.NET (1.0 and 1.1) were criticized for their lack of standards compliance. The generated HTML and JavaScript sent to the client browser would not always validate against W3C/ECMA standards. In addition, the framework's browser detection feature sometimes incorrectly identified web browsers other than Microsoft's own Internet Explorer as "downlevel" and returned HTML/JavaScript to these clients that was crippled or broken. However, in version 2.0, all controls generate valid HTML 4.0, XHTML 1.0 (the default) or XHTML 1.1 output, depending on the site configuration. Detection of standards-compliant web browsers is more robust and support for Cascading Style Sheets is more extensive.
ASP.NET 2.0 produces markup that passes W3C validation, but it is debatable as to whether this increases accessibility, one of the benefits of a semantic xhtml page + css representation. Several controls (eg. the Login controls, the Wizard control) use html tables for layout by default.
Microsoft has now gone some way to solve this problem by releasing the ASP.NET 2.0 CSS Control Adapters, a free add-on that produces compliant accessible xhmtl+css markup.