Microsoft recently had a high-profile launch of its new operating system, Windows 8, which drastically changes how Windows users will interact with their computers and other devices.

But unexplained in the commercials is why there’s Windows 8 as well as a separate new piece of software called Windows RT that was rolled out at the same time. What is Windows RT and how is it different?

While Windows 8 is meant to power desktops, laptops and other systems using hardware from companies like Intel and AMD, Windows RT is meant for mobile devices running processors from a company called ARM.

Windows RT won’t run traditional desktop applications from the Windows world like Photoshop, the desktop versions of Microsoft Office or anything else that wasn’t built for Windows 8. Instead, RT will run mobile apps specifically built for that platform.

The first major device running Windows RT is Surface, a Microsoft-built tablet that launched on Oct. 26 and starts at $499. It comes with a suite of apps for email, web browsing and other functions and runs mobile versions of Microsoft software like Word and Excel.

A pricier version of the Surface tablet, running a full version of Windows 8 and powered by an Intel processor, should be out in a few months. It’s expected to run all Windows 8 applications, not just ones built for mobile devices.