We've just seen how ASP.NET works from a high level. When a client surfs to an ASPX file on your server, the request is pushed through IIS running on your server. IIS maps individual file extensions to specific ISAPI DLLs. When IIS sees the .aspx extension in the request, that ISAPI DLL is aspnet_isapi.dll. The request ends up within the ASP.NET worker process, which instantiates an HTTP handler to fulfill the request.
In the case of an ASPX file, ASP.NET instantiates a class derived from System.Web.UI.Page (which implements IHttpHandler). ASPX files are usually paired with source code files containing the source code for the page. The ASPX file behaves mainly as the presentation layer while the accompanying Page class contributes the logic behind the presentation layer.
Next up—all about System.Web.UI.Page and how Web forms work.