Many window managers and graphical environments provide "virtual desktops" which allow the user to spread open windows out among multiple virtual workspaces. The user can then switch from one workspace, or desktop, to another with only one or two windows open on each one. This feature is a luxury that quickly becomes a necessity once you've used it for a while.
Unfortunately, Aqua does not have native support for virtual desktops, making the transition from an X11 window manager such as WindowMaker more painful than necessary. Fortunately, there are a couple of third-party virtual desktop applications.
Space.app is an open source virtual workspace implementation distributed free of charge under the terms of the QPL. It has limited functionality and although it has potential, it is not quite ready for prime-time IMHO.
I've settled on CodeTek's VirtualDesktop although it is closed source and commercial software. VirtualDesktop has all the features you could ask for including a transparent, skinnable pager that has mini-window representations of the open windows on each desktop. For me, the key feature is the ability to bind hotkeys to jump directly to a specific workspace. It is also possible to bind a hotkey to hide all desktop icons. Clear that virtual clutter!
As you may have guessed by reading some of my previous entries, I am obsessed with keyboard shortcuts. Dave Polaschek has compiled a table of many useful key sequences for the mac.