Windows RT is a sort of unique member of the Microsoft family of operating systems. It’s Windows, but it’s not. From a more technical aspect, Windows RT is actually the entire Windows OS rebuilt for use on ARM processors, which are what power almost all tablets, smartphones, and other gadgets.
This means that you have Windows technically, but it also means it can’t run regular Windows desktop applications. However, Windows RT can run certain Windows programs made for ARM, and while you could toy with the Windows Store, another thing you can do is unlock your Windows RT device to be able to run third-party programs.
Don’t let a limited Windows Store and different architecture fool you, Windows RT really is full-blown Windows 8 made for ARM devices. However, Microsoft did actually leave a security lock on the OS much like Apple’s own iOS lock, meaning you can’t run unsigned, third-party programs, even if they have been compiled for Windows RT.