Inside Mac OS X 10.7 Lion: New Dock, Finder & Desktop
Automatic Spaces and Mission Control
Apple indicated that it worked hard in Leopard to deliver a new virtual desktop feature without introducing too much additional complexity. The problem, however, is that virtual desktops are innately confusing for most users because they act as modal representations of parallel worlds, each with its own desktop of icons.
Apple's implementation, named Spaces, shares the same desktop for each Space, merely grouping a set of app windows (or apps themselves) into different screens. Spaces work for power users who switch between different types of tasks, but they're confusing for most users because there's an explicit, invisible step required to move between each space.
Mission Control, a renaming and enhancement of the F9-invoked "all windows" Exposé screen, simplifies Spaces by making it visually simple to see each space in the context of running apps and all document windows. An important part of this simplification is the use of multi-fingered gestures to navigate through the screens visually. This makes Spaces much easier to visualize and navigate between.
On page 3 of 4: Full Screen Apps.