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DotNetNuke handles additional functionality through the use of extensions. It is possible to create your own extensions for the platform using a wide variety of tools.
This page will provide extension development information.
There are a number of ways to get started with extension development.
Developing Modules can be done in a number of ways, using both VB.net and C#. Check out the Module Development page
Developing Skins
Developing Modules
Developing Providers