The "RT" in Windows RT stands for RunTime. It is because Windows RT is designed to run on lower power ARM processor tablets. A smaller runtime version of Windows 8 will be used, hence the name. Windows RT won't have the power of a full Windows 8 capable Intel x86 processor computer. But the Windows RT models will be cheaper and capable of doing the same as iPads and Android tablets today.
NOTE: I wrote this in July 2012. Windows RT, 8 and Surface are due soon, August or September 2012.