The first release of Microsoft’s Suface Tablet, cost about $499, comes with Pre-installed Windows RT. It is (i assure you) REAL windows, but only just; its more like a Mobile OS. Windows RT has the same interface as the one you get in windows 8 pro, and it fully supports all the windows 8 apps.
But what it does lack, is the support for Desktop apps, Means, you wont get the desktop interface in it, nor will the OS support programs that were designed for previous versions of windows(the desktop ones.) While Windows 8 Pro supports both. Another difference between the two tablets is that the RT one runs on an ARM Processor while the other one uses INTEL.
SO, the main difference between the two is that RT supports only METRO apps, while PRO supports METRO+DESKTOP apps, and runs on a comparatively better processor.