Spaces, introduced in OS X Leopard, gives you virtual desktops to organise running apps. OS X Lion improves Spaces with the introduction of Mission Control and full-screen apps. Together with new trackpad gestures, Spaces is now an integral part of how you interact with apps in OS X Lion.
When you turn an app to full screen mode, OS X Lion will put the app into its own Spaces. To switch between Spaces, you swipe on the trackpad left or right with three fingers. You can add or remove Spaces directly from Mission Control (run the Mission Control app or swipe up with three-fingers on trackpad).
By default, Dashboard appears as the first Spaces. If you are not a Dashboard user, you can hide Dashboard from appearing as Spaces.
In OS X Lion, Spaces is part of Mission Control. Open the Mission Control settings in System Preferences to disable Dashboard as a space. Uncheck the settings “Show Dashboard as a space” to hide Dashboard as Spaces.