WHAT IT IS: A replacement to e-mail? Well, that's an ambitious goal, but it's the idea behind Google Wave. It's what you'd get if you married e-mail with instant messaging with a wiki. Google Wave's creators argue that e-mail and instant messaging are just the electronic version of snail mail and phone calls -- and that's limiting their potential, making them not really as collaborative and media-rich as they should be. Instead of sending an e-mail back and forth and everyone getting copies of the communication, a wave is hosted in the cloud, and anyone you invite can work within it in real time, adding video, images and digital tools.
EXAMPLES, PLEASE: As an illustration of e-mail's shortcomings that Google thinks Wave can fix, consider trying to take a poll in e-mail. What you get back is a bunch of separate responses that you then have to collate and tally. With Wave, all the poll responses would live in one place and there'd be no need to aggregate a bunch of answers. Google has made Wave open-source, so developers can create apps and widgets (such as polling apps) that can be embedded into the waves.
POTENTIAL USES: A few ideas of what Wave might be good for include collaborative reporting (as the LA Times notes, double bylines aren't common because they're actually a pain to execute but Wave makes sharing notes and real-time collaborative writing/editing easier); building presentation documents; or planning a trip with another person.
THE FINE PRINT: There will certainly be a network effect at play -- the more of your colleagues and friends using Wave, the more useful it will be. However, anyone who wants to use Google Wave will need access to the product and right now, given that Google has only dropped 100,000 invites, don't expect it to be of much use. But remember, that's how Gmail started out.