ASP.NET AJAX is a set of technologies that allows developers to create interactive web applications. It enhances the user experience by enabling partial page updates, meaning only specific parts of a web page can be refreshed without reloading the entire page. This leads to faster and more responsive applications. With ASP.NET AJAX, developers can easily integrate client-side scripts with server-side code. It provides tools like UpdatePanel and ScriptManager to manage asynchronous requests, making it simpler to build rich, dynamic web interfaces. Overall, it streamlines the development process and improves application performance.