Back in the early stages of Windows 8, Microsoft announced that there would be 4 different versions of Windows 8. Windows 8, Windows 8 Pro, Windows 8 Enterprise and Windows RT. The first 3 are fairly obvious as to what the differences are but what's Windows RT all about? Not everyone knew about this "RT" version of Windows 8 until Microsoft revealed their Surface Tablets.
Essentially, Windows RT is a special version of Windows 8 that you won't be able to buy off the shelve. It will only come pre-installed on devices that have ARM based processors instead of the traditional x86 Intel and AMD processors that we are all used to. Since computers really began taking off, they've all been powered by the traditional x86 processor architecture. ARM processors have been around for a while but are more favoured for low powered devices like mobiles etc..
ARM processors consume much less power than x86 processors but also produce less power. However over the last few years, ARM processors have advanced dramatically and now many of them can match x86 processors, all the while being much more energy-efficient. As a result they are becoming more popular in larger devices such as tablets. Not only because of their high processing power and low power consumption, but also because they are generally cheaper than x86 chips.