Windows 8 is the standard version, Windows 8 Pro has some more features for power users, and Windows 8 RT — the RT is short for "Runtime" — is completely new and completely different.
Windows RT vs Windows 8: upgrades and boxed versions
Windows RT is only available pre-installed on devices. Windows 8 will be available pre-installed on new PCs, as stand-alone retail boxes and as upgrade versions too. Microsoft has been pushing a Windows 8 upgrade offer.
Windows RT vs Windows 8: price
Windows RT is literally priceless, because you can't buy it, but licensing fees to manufacturers should be significantly less than for Windows 8 or Windows 8 Pro. Windows RT devices will cost less than Windows 8 ones - think Android tablets or the iPad versus Windows laptops, Microsoft Surface starts at £399 — and Windows RT users won't have to pay for their copies of Office (Word, PowerPoint and Excel only, though).
Windows RT vs Windows 8: upgrading
This one's easy: you can't upgrade to Windows RT from anything, because it's a brand new Windows for a previously unsupported platform.
If you're currently on Windows 7 you'll be able to upgrade to the normal Windows 8 from Starter, Home Basic or Home Premium, but you'll need to perform a clean installation if you're running Windows 7 Ultimate or Professional. If you plump for Windows 8 Pro, you can upgrade from any version of Windows 7. As ever, 32-bit installations can only be upgraded to 32-bit versions, and 64-bit ones to 64-bit ones.
Windows RT vs Windows 8: hardware support
This is the most important difference between Windows RT and Windows 8: Windows RT only runs on ARM-powered devices, while Windows 8 only runs on x86 devices. Windows RT won't run natively on an Intel- or AMD-powered PC, and Windows 8 won't run on an ARM-powered devices such as the new Surface tablet.
Windows RT vs Windows 8: interface
Both Windows RT and Windows 8 run the new Metro interface, but Windows 8 can also drop down to the traditional Desktop for older apps. Windows RT can't in the same way: while it does have the old Windows Desktop for some of Microsoft's own apps, the desktop won't be available to third-party software.
Windows RT vs Windows 8: software and compatibility
There's a big difference here: Windows RT comes with Office preinstalled, and Windows 8/Pro doesn't. However, Windows 8/Pro are compatible with existing Windows programs and Windows RT isn't. Windows RT apps must use the Microsoft-style interface, although Windows 8 programs can use the traditional Windows desktop too.
Windows RT also restricts what APIs (application programming interfaces) developers can use, especially for web browsers. As Mozilla's Harvey Anderson puts it, "only Internet Explorer will be able to perform many of the advanced computing functions vital to modern browsers in terms of speed, stability, and security to which users have grown accustomed."
Windows RT vs Windows 8: features
As you might expect, Windows RT doesn't get everything you'll find in Windows 8. There's no Windows Media Player or Storage Spaces, no Bitlocker encryption (although Windows RT does offer device encryption instead), no Group Policy management or domain support, and Remote Desktop only works in client mode.
The differences aren't as dramatic as you might expect, however: Windows RT and Windows 8 both get multiple language support, IE10, Xbox Live, Windows Defender, Exchange ActiveSync, Windows' Play To streaming and virtual private networking.