Windows 8
Retail versions of Windows 8 are only able to install these apps through Windows Store—a namesake distribution platform which offers both apps, and listings for desktop programs certified for comparability with Windows 8. A method to sideload apps from outside Windows Store is available to devices running Windows 8 Enterprise and joined to a domain; Windows 8 Pro and Windows RT devices that are not part of a domain can also sideload apps, but only after special product keys are obtained through volume licensing.

Q: Where can Windows 8 install apps from?
Windows Store