DotNetNuke, often referred to as DNN, is a web content management system. It is a tool designed to be used from a web browser that can create any type of website, from small business sites to online communities and intranets.
From the user's point of view, using DotNetNuke is like connecting to Hotmail to manage emails, or connecting to Facebook to post pictures and messages: you connect to DotNetNuke to create pages and manage your website's content.