ASP.NET is a web application framework built on the .NET platform. It allows programmers to build dynamic web pages and web applications using any .NET language like C# and VB.NET. ASP.NET files have the .aspx extension and can contain HTML, XML, and server-side scripts that execute on the server. When a client makes an HTTP request for an ASPX page, the web server executes the page code and returns dynamic HTML, CSS, and JavaScript to the browser.