ASP.NET was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft’s Active Server Pages (ASP) technology. ASP.NET is built on the Common Language Runtime (CLR), allowing programmers to write ASP.NET code using any supported .NET language.
Typically, ASP.NET refers to web forms style of development, which is the original method of developing web applications in ASP.NET. Web forms is a model which mimics the control/event model of classic WinForms development, providing the illusion of stateful pages.
However, in March of 2009 Microsoft released version 1.0 of the ASP.NET MVC Framework. As an alternate model to web forms, this framework applies the Model-View-Controller pattern to web applications in ASP.NET.
As ASP.NET includes its own controls library, wrapping HTML controls and adding functionality to them, any ASP.NET controls have as markup <asp:ctrl_name></asp:ctrl_name>.