It's a single window, that displays another single window, keeping it on top. Great for watching stuff like TV or Youtube playlists in a small corner of your workspace while working in the rest of it. It's also great for tasks that would otherwise require flicking back and forth between windows, reading documentation and key tapping in the IDE for instance.
As you can see from above, you can zoom into specific areas, repeatedly if so desired. After that you can resize the window to blow up that specific portions way beyond normal size:
Of course, passive viewing by itself isn't brilliant. To address this WindowWatcher allows interaction too, anything you could do with the mouse and keyboard in the original window you can do via it. Posting to Twitter or Facebook, STFW-ing and faffing with WMP playlists being my current poisons:
Topping it off there's a simple plugin API for those stubborn windows which don't react normally to window messages.
Those windows better watch out, there's a new stalker in town.