We've already established that one of the Surface's most distinctive features is its Windows RT operating system, designed to run on hardware driven by ARM's processor architecture. Experientially, Windows RT is very much like the x86 version of Windows 8 we reviewed in The Definitive Windows 8 Review And User Guide, sharing the same Windows 8 UI, Start screen, and multi-touch gesture support.
To quickly recap, swiping from the Surface's right edge reveals the Charms bar, swiping from the left cycles through the most recent apps, and swiping in and out from the left edge actives the Switcher. This is no different from how it will be on a Windows 8-based tablet.
Windows RT handles multi-tasking much differently than other mobility-oriented operating systems. iOS- and Android-based tablets suspend background applications, forcing you to use one at a time. In the screenshot above, you can see that we're in Windows RT's Desktop app using Word, Excel, and IE10 simultaneously.
Of course, there are notable differences. For example, Windows RT does not have a Windows Media Player. Instead, there's a dedicated Video, Music, and Photo app to handle those tasks.
Moreover, Windows RT ships with Office 2013 Home & Student RT, which includes Word, Excel, PowerPoint, and OneNote. Those apps are functionally similar to what you'll find in the Customer Preview of Office 2013 for Windows 8. The changes specific to Windows RT largely center on optimizations for performance and battery life. For example, the Surface's copy of Office displays a non-blinking cursor once you go idle, since the animation drains resources. Also, GPU acceleration is enabled by default. All Windows RT-based devices must satisfy a minimum specification, guaranteeing hardware support for certain tasks (like playing embedded video within a slide). Because desktop PCs are more diverse, Microsoft isn't able to impose the same requirement, shifting playback to the host processor instead.
But the most significant difference between Windows 8 and RT is that the Surface's operating system is only compatible with applications that come from the Windows Store. If you try to install Firefox, you'll get an error. No third-party apps will run on the Windows RT Desktop.
Can you at least hope to see the same long list of applications compatible with Windows RT as Windows 8? Unfortunately not. Apps written in .NET are platform-agnostic, allowing them to run under both operating systems. However, software written in C++ would need to be recompiled for the ARM instruction set.
For more information about the compatibility story, check out Windows 8: Clarifying Codecs, Compiling, And Compatibility.