Plenty of tablets let you run multiple applications at once, but Windows 8 is the only OS that lets you see more than one of those apps at the same time, including multiple Metro and desktop apps. This is far more flexible than it first looks (and it’s not quite the same as running multiple programs on the Windows desktop).
You can run a Metro app or a program on the Windows desktop full screen and treat it as the only application you’re interested in. If you do that, other desktop apps carry on running in the background the way you’re used to and you can switch between them as usual.
But Metro apps don’t keep running if you can’t see them on screen; they’re suspended automatically after a few minutes and resumed when you switch back to them. In the developer preview that works pretty well most of the time, but this is very early code and it does have bugs; we have seen apps hang when we tried to switch back to them.
If you don’t want just one app to fill the screen, you can have what Microsoft calls a ‘full’ window for the app and a tall, thin ‘snap’ window for another, and you can drag the divider between them to swap which window is full and which is snapped.