An ASHX file is a web handler file used by the Microsoft ASP.NET framework to process specific requests on a web server. Unlike standard ASPX files, which are primarily used to generate HTML web pages for user interfaces, ASHX files are designed to serve dynamic content that does not require the overhead of a full web page lifecycle. They are commonly used for tasks such as generating images on the fly, returning XML or JSON data for AJAX requests, handling file uploads or downloads, and creating custom RSS feeds. ASHX files implement the IHttpHandler interface, which allows developers to write efficient code in languages like C# or VB.NET to respond directly to HTTP requests. When a client requests an ASHX URL, the ASP.NET engine executes the logic contained within the file and sends the output directly to the client's browser or application. This makes them a lightweight and high-performance solution for backend web processing tasks that do not need a graphical user interface.