Basically, when the Waveboard is open, what you see mirrors the page you see on your browser. The two major differences are firstly, that Waveboard is clean of all the other interface distractions that your browser has, and secondly that it has an icon that sits in your dock and notifies you when there is something happening in Google Wave.
Currently, with the web app, unless you're logged in and watching, you can miss things, but with Waveboard the experience is more like a desktop messenger app. To use all the features you will have to have Google Gears installed, but that's something Mac users should already have!
As Google Wave itself is still in beta, there are likely to be problems that need ironing out, but in our tests it seemed perfectly stable and just as functional as using your browser. Whether a desktop client turns out to be a useful way of using Wave depends really on how usage develops. Waveboard and Google Wave are so new that it's hard to say how we'll be using it in a few months from now.
There are a couple of functional advantages to Waveboard, especially in terms of notifications, but only time will tell if it fits with how users interact with Google Wave.
- This new version again tries to make the use of Waveboard on Mac more convenient.