DotNetNuke is an open source web portal framework built on the Microsoft Asp.Net technology, earlier version used Asp.Net Version 1 and newer versions use the Asp.Net Version 2 framework.  It is open source because the source code for the framework is freely provided, it can be inspected and modified by anybody.  There are many such applications available.  These include operating systems (OpenBSD, Unix) to GIS (Geographical Information Systems) software and many other web applications. 

DotNetNuke has gone from a little known project to the mammoth project that it is now.  There are many people who have contributed and modified the application plus a core team of developers that dedicate generous amounts of their time to ensure the continued development and improvement of the framework. It is robust and has a multitude of plug-in modules available.  Part of its success is with the ease of development of custom modules.  These can provided freely or require payment for use.  Only the DotNetNuke framework is provided free of charge.

The DotNetNuke framework isn't just used by sites on the World Wide Web, it can be found in companies on their internal network and anywhere else there is a Windows based network.