Basicly there is a groundsystem of DNN which presents the Basic functions and can be used as a Framework This is the DotNetNuke if self.  The Basic System is updated from DNN by new Versions to fix issues and make enhancements.  So you have to know what Version are in use, you can check this on your site on the system setting.

Custom Enhancements should are made as a module, provider, widget or skin, that packaged as a zip file and could be installed by uploading as a Systemadministrator of the DNN instance.

Content are managed over the DNN site. 

If you Need to develope enhancement you need to know for what DNN Version and you should then create a local instance on the developer Maschine using the starter package on the Version you needed.  Of cause you also can use the newest Version for developing, but you have to know that the min. Version your production page is running must be have the Version with the enhancement was created with.