An aspx file is a file with an .aspx extension on your computer that contains programming statements written in server scripting, also known as ASP or Active Server Pages.
An ASP page can have HTML code like other pages, but it will often contain Active Server Controls (ASCs) to provide additional functionality. This code can be “clicked” instead of using a submit button. In order to make this work properly, the HTML form needs validation so that certain fields are not submitted until they are valid. These ASCs and other coding help provide advanced user interface feedback when errors occur in various parts of the input processes provided by forms and other web-based input devices employed on an ASP site.