DotNetNuke is a program(framework) that runs on Microsoft ASP.NET. It is a open source framework which  is designed to be extended. Anyone can freely use this framework and add additional modules on it. One of the ways you extend the framework is to create modules. These modules are installed inside a DotNetNuke installation and when they run in that DotNetNuke installation they extend the framework to create a DotNetNuke website also called a portal. Since it is a open source application you can find lots of modules contributed by different enthusiastic developers.

A single DotNetNuke installation will allow the creation of thousands of individual portals. Only limitation is up to the servers hardware configuration.s DotNetNuke portals are configured to display pages and the pages are configured to display modules.

The portal is accessed by users via a web browser (this can be over the internet or an intranet). These users can be either anonymous visitors or users with accounts and passwords. The modules that are installed and configured in the DotNetNuke installation provide the functionality that the users can perform.

One of the most beautiful thing of dotnetnuke is it supports skinning. You can change the the design and layout of the portal using  skins which contains skin objects css and images.