In my Scientific American column this month I explained why touch-screen desktop computers are annoyingly hard to use. (It has something to do with so-called "gorilla arm.") But leaving aside those problems, Microsoft's Windows 8 operating system might confuse longtime PC fans, at least at first. That's because it feels like two different operating systems—one for mouse-and-keyboard PCs (the traditional Windows desktop) and one for touch screens (a new mosaic of bright, colorful tiles). Each has its own way of doing things. And each runs its own kind of programs.

Actually, that's not the only confusion. Some computers will be able to run both kinds of programs (traditional Windows apps and the new tile-type apps) and some will be able to run only the new tile-type apps.

What the world really needs is a glossary to help keep this all straight. Here it is. Clip, save, enjoy.

Windows desktop apps: These are the regular PC programs you've used for years: Photoshop, Quicken, Excel, games. Now that there are two different kinds of programs, it's important that the old kind have a name.

Windows Store apps: This new kind of app is geared toward the tile-based touch-screen environment. These apps fill the screen, don't have traditional drop-down menus, and generally aren't as ambitious or powerful as regular PC programs. Microsoft supplies Windows Store apps called Calendar, Mail, Photos and Music; the store offers many more iPad-style apps. Just be careful (see next item).

Windows Store: Somewhat confusingly named, the online Windows Store sells both Windows Store apps (it's their only source) and traditional Windows desktop apps (it's not their only source).

Windows RT: An operating system that does not run Windows desktop apps—only Windows Store apps. That's because Windows RT (short for "runtime") does not run on Intel chips—only on so-called ARM chips, which are lower-power, longer-battery-life chips found in tablets and some laptops. A Windows RT tablet such as Microsoft's Surface tablet runs only Windows Store apps.

Windows 8: Windows 8 is the full Windows version, the one that requires Intel chips and can run both Windows desktop apps and Windows Store apps.

Windows 8 Pro: This more expensive version of Windows 8 contains a few additional features aimed at power users and corporate workers.

Metro: Microsoft gave the new Start screen (the screen full of colorful tiles) the code name "Metro" during development. Microsoft doesn't use that term officially, but it hasn't given us a replacement term to refer to the new tile-based interface.

Surface: That's Microsoft's own $500 touch-screen tablet. It's a Windows RT device, meaning that it doesn't contain an Intel chip and runs only Windows Store apps.

Surface Pro: Microsoft also sells this much more expensive, somewhat thicker and heavier tablet that does contain an Intel chip and does run both Windows Store apps and traditional Windows desktop programs.