The prayers of baffled beginners and exasperated experts everywhere have now been answered. Now, when the novice needs help from the guru, the guru doesn't have to run all the way downstairs or down the hall to assist. Thanks to the new screen-sharing feature of Leopard, you can see exactly what's on the screen of another Mac, from across the network —and even seize control of the other Mac's mouse and keyboard (with the newbie's permission, of course).
(Anyone who's ever tried to help someone troubleshoot over the phone knows exactly what this means. If you haven't, this small example may suffice: "OK, open the Apple menu and choose 'About This Mac.'" Pause. "What's the Apple menu?")
Nor is playing Bail-Out-the-Newbie the only situation when screen sharing is useful. It's also great for collaborating on a document, showing something to someone for approval, or just freaking each other out. It can also be handy when you are the owner of both Macs (a laptop and a desktop, for example), and you want to run a program that you don't have on the Mac that's in front of you. (You might want to adjust the playlist selection on the upstairs Mac that's connected to your sound system, for example.)
Or maybe you just want to keep an eye on what your kids are doing on the Macs upstairs in their rooms.
The controlling person can do everything on the controlled Mac, including running programs, messing around the folders and files, and even shutting down the controlled Mac.