"Modules" are modular pieces of code that each present some functionality to the user.  Popular modules include user forums, photo galleries, blog publishers, directory tools, chat applications, contact managers, and events calendars. 

In typical use cases, several modules will be grouped together to create a webpage or intranet page.  These modules can easily be added, edited, deleted, restored, moved within a tab, or transfer to another page.

Within the DotNetNuke ecosystem, there are a wide variety of developers continually producing new and exciting modules for the DotNetNuke application framework. These module developers often build modules which they will sell on their sites or through marketplaces or reseller stores (some offer modules for free.)  Developers will often offer up their services to design custom modules for their clients.