No, this isn't about running out of room for your Sunday School classes, this is about a new feature in Mac OS X Leopard (10.5).
Spaces is like having 2, 4, 6, however many desktops. I didn't think much of it when I saw the previews but after using it, it really is a productivity improver.
Unless you have a really big monitor, Spaces can really help you to manage all those open windows that you like to have but not sure what to do with.
Imagine that on one screen you can be running your email program, another you can be running iTunes listening to music or the latest podcast on accelerating snail growth, and on the third screen, you have your church management program and browser open.
With a simple key combination, you can switch to these various screens to maintain a clean, uncluttered look.
Sure, you can use Expose to navigate your application windows but if you have more than four or five open, even that can get unwieldy, especially on a laptop monitor.
Spaces is like Expose on steroids - now you're seeing desktops in scaled windows, not just applications.
Now before you go bragging to everyone, having multiple desktops has been around for a while, Apple's made it accessible, usable, and intuitive.