ASP.NET is a web development platform that provides the programming model, infrastructure, and services needed to build robust web applications for desktops and mobile devices. It uses HTTP to enable two-way communication between the browser and server. ASP.NET applications are compiled code written using reusable components in the .NET framework and can be written in languages like C#, VB.NET, JavaScript, and J#. ASP.NET is used to create interactive, data-driven web applications accessed over the internet using various server controls.