What is DNN?
DNN stands for "DotNetNuke". A platform created back in 2002 as a fork of Microsoft's "IBuySpy Portal" project. The latter was an asp.net solution which aimed to show how one could build a rudimentary CMS with this new technology. Since those days DotNetNuke has exploded onto the CMS scene. To date it is the most successful CMS built on the Microsoft "stack" (Windows Server, .net framework, SQL Server). There are thousands upon thousands of websites that operate today based on this CMS. And with today's feature set you can build very complex web-based solutions.