In terms of looks, Windows RT and Windows 8 use the same new-style Start screen. However, Windows RT has been designed specifically to run on ARM-based tablet devices. Traditional computers run on CPUs that feature the x86 architecture, but many tablets and low-power computers run CPUs that are based on the ARM architecture.
You can't run Windows 8 Pro on devices that have ARM chips, and you can't run Windows RT on devices that have x86 chips. ARM chips consume less power than x86 chips, which means they also produce less heat and don't chew through battery life as fast. They are favoured by manufacturers who want to create small and thin devices, including phones and tablets.
To cater to the market for thin and light devices, Microsoft released Windows RT. It will be available on the entry-level Microsoft Surface tablet, which runs the NVIDIA Tegra ARM-based CPU and it has a starting price of $559. It will perform similarly to Android-based tablets, which also run on ARM CPUs. Windows 8 applications won't be able to run on Windows RT, so you won't be able to install a traditional copy of Microsoft Office, for example. Instead, there will be a special version of Office 2013 that will be available for Windows RT and it will include Word, Excel and PowerPoint applications.
The high-end version of the Surface tablet will run a third generation Intel Core i5 CPU and it will be able to be used as a laptop when resting on a desk, in addition to a tablet when you are on the move. It will run traditional applications without any problems and will offer a similar level of speed as a conventional Ultrabook, for example.