Visual studo is used for the development of modules that "plug" into the DNN framework. You can even alter the framework with Visual Studio although these hacks are not recommended unless you really need to go this way, or you wish to bend the framework for a seperate project.

Pretty much anything can be managed without visual studio (even module installation)... Dotnetnuke has a permissions based model which disables or enables certain functionality depending on how the user security is set up... there is simply no point in using VIsual Studio for these purposes.

Dotnetnuke supports skinning - you don't necessarily need to use Visual studio for this - you can by third party skins to plug in, or even use Dreamweaver to develop them.

The best way to understand it is to install (DNN. 5.1.0 should be out soon).... plenty of documentation on how to do is available (google it), although if you a used to installing ASP.NET apps it shouldn't be too much of a problem.