You can download the current version of DotNetNuke from this link. I became familiar with DotNetNuke when it first was developed from the IbuySpy portal. The early versions gave you a quick way to setup a portal on your web using the new .NET Framework 1.0.  The benefits were a quick way to setup your web site, with the ability to register users baked in; no programming was necessary. Also, there were announcements and the ability to allow end users to edit content, again without programming.

Developers could modify the source code of DotNetNuke to customize it how they wanted.  While it is programmed in VB.NET, C# modules can be created. Developers could create custom modules containing the look and feel of the portal. Changing that look and feel was not an easy change but it could be done site wide. The main power of the original DotNetNuke, I believe was the ability to create separate portals within one DotNetNuke application with different content.  The original IBuySpy would not easily let you do this without modifications to the code.

Originally, DNN supported SQL Server as the backend. As new versions have been created, DNN has created more ways to connect with other data stores. Later versions also make customizing the look and feel of a portal an easier task.

The current DotNetNuke has taken these concepts much further, and in a slicker package.  Besides more modules to choose from in the default package, the design in DNN has progressed also. For developers, they are using a Provider Pattern for data access.  If you need details on that feel free to look at their documentation found here.