Read more at: Microsoft: We aren't going to have three different versions of Windows | ZDNetMicrosoft currently has three different versions of Windows running on mobile devices. But Microsoft is working to reduce that number, Julie Larson-Green, Executive Vice President of Devices and Studios, confirmed in an interview last week.
At present, Microsoft offer plain-old Windows for Intel-based PCs and tablets. There's Windows RT for ARM-based PCs and tablets. And there's the Windows Phone OS for Windows Phones.
I reported earlier this year that one of my sources said Microsoft is planning to whittle this down to two versions, though possibly not until the spring of 2015.