Start screen only appears on your tablet (or smartphone?)
Tablet acts as a glorified app launcher
Desktop applications have metro app counterparts
Metro apps do what they're good at...allow you to consume content and provide a high level overview of your data. When you want to drill in and produce/modify content, options within the metro app will launch the associated desktop application on your PC screen.
Metro app extends desktop application by providing relevant info and options as you use the application
Quick, rough example...you listen to Xbox Music on your tablet and you want to manage your music library (edit meta data, etc). You click the option in the app and the Xbox Music desktop application (I don't know why this doesn't exist yet) launches. You would then swipe through your collection on your tablet (Xbox Music) to pick out the album you want to modify (in desktop). Music controls would appear on the tablet to control any content you play.
Another brief example...Visual Studio - class/solution overviews & tfs management on your tablet?
This is sort of similar to a tablet docking scenario in which the tablet is the primary monitor, except that metro apps and desktop applications would talk to each other directly. So, basically the canonical PC+ scenario where the tablet extends your desktop work.
And of course, the existing Win 8 form factors (touch-enabled ultrabooks, touch all-in ones) would stay. This would be in addition to all of those.