Sideload in Windows 8
In Windows 8, there are two type of interfaces. One is traditional interface, another is Modern (also called Metro). You can only install a Metro application from Microsoft Store.
Obviously, you can't depend on Windows Store in a enterprise environment. So Microsoft provides a few ways to IT Pro to deploy internal apps in Active Domain environment. This called Sideload.
The apps waiting for deploying still have to have a trusted certificate. In addition, Microsoft recommends IT Pro use Windows App Certificate Kit to check the apps before deploying them.
So, IT Pro can use Deployment Image Management DISM to deploy a private Windows 8 app in a Active Domain. Alternatively, Microsoft Deployment Toolkit 2012 Update 1 MDT can do the same job. IT Pro only need to create a new application in MDT.
And of course, IT Pro still can go to a Windows 8 desktop and install the app manually. What he need is the app source file and a installing script written in Powershell.