Content Types, simply put, are a way of telling the framework what module/functionality is associated with a Content Item. Each product (ie. module) that wishes to allow categorization of data (via Taxonomy or Folksonomy) for it's content items will likely need to create its own content type.

In the core forum module, for example, the forum creates each thread as a content item. All posts in a single thread are part of that content item. This also means that a single forum module instance will contain multiple content items. Because of this, the module required it's own custom content type. The core skin object, which doesn't actually create any content, utilizes the 'Tab' content type that exists as part of the framework.

Default Content Types:

• Tab
• Module
  

Additional Content Types:

• DNN_Forum - The DotNetNuke Core Forum module.
  

In situations where you don't create a content item, like a skin object