DotNetNuke is a program that runs on Microsoft ASP.NET. It is also a framework, meaning, it is a program that is designed to be extended. 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.
A single DotNetNuke installation will allow the creation of thousands of individual portals (as much as the server hardware can handle). 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 a 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.
The DotNetNuke framework provides the various functions such as determining if a user is logged in, displaying the look and feel, determining what is on a page and who can see it. The DNN module extends the framework and allows users to participate and use the custom logic defined at DotNetNuke Custom Modules.