Okay, get some popcorn and sit back in your favourite chair for a little more than an hour presentation of Google Wave!
If you're too impatient to sit through the presentation, then you can read this blog article by Tim O'Reilly (of O'Reilly Books), which summarizes some of the key features (however, it's much better to see the whole video presentation, of course).
This could very well be the way online communication is handled shortly, so it could be bye bye e-mails, IMs etc.
Not only that, but with more extensions, developed for the open source platform, it could even be used as a handy Virtual Table Top. Just imagine something like RPTools' MapTool as an extension!