Even the largest monitors can be overwhelmed with open windows.
New in Leopard, Spaces solves this problem by allowing the user to define multiple desktops. You might have one desktop for browsing and research, one for photo editing, and one for working with music and movies, with appropriate icons placed on each.
Spaces makes it easy to see all your desktops, to navigate among them by tabbing or clicking, and to drag and drop files from one desktop to another.
Applications open in any of your Spaces are also represented on the Dock; just click the icon, and you'll be transported to the related desktop automatically.