What are ASP.Net Pages ?

ASP.Net pages can be identified by the file extension .aspx. Conceptually they work along the same lines as conventional ASP pages although the whole process is applied to the .Net Framework. In an ASP.Net page the code is separated from the document mark up; this simplifies the layout and understanding of the document. Place holders are used to determine where areas of active content should be inserted into the document by the web server. The code which has now been separated from the document mark up is then stored either at the top of the document within script bocks or preferably in a separate file called a Code Behind File. Separating the code into code behind files allows the code to be compiled into a single file stored on the web server ready to service any web requests.

The process for viewing an ASP.Net page is that the end user requests the web page (in this case the ASP.Net document) using a web browser. The request is sent by the web browser across the internet to the web server. The web server receives this request, identifies the document is an ASP.Net document and gathers any dynamic content from the precompiled file. Any instructions are processed in accordance with this file and the results rendered as HTML which is inserted back into the document where the placeholders indicate. This new document is passed back across the internet to the web browser. The web browser receives this file and displays the document content in accordance with the instructions contained within the HTML.
One of the great strengths of ASP.Net is that the platform it is capable of rendering different HTML code according to the version and type of browser that made the web request. So ASP.Net can generate different HTML code for different circumstances and this is completed without intervention by the programmer. In theory the programmer prepares the website content and instructs ASP.Net how it should be displayed and then the code generated should be correct for the users platform, whether a standard computer or a third generation mobile telephone device. In practise this is still in its infancy and a programmer needs to be aware of what code will be generated and for what platforms.

More at http://www.codefixer.com/asp-net/tutorials/what-is-asp-net.asp