You can define it as an "email" solution if email was invented today or as an instant wiki communicator.
You can share your "email" conversation (a wave) with all your friends/colleagues/network, instead of sending individual emails to predefined people.
You can just reply to someone like replying to an email, but as replying contextually within the text posted by the others (like editing a wiki page) and you see all changes in real time!
Furthermore, later on, you can easily play the recorded "real time" responses chain to better follow or understand a previous wave.
Last, but not least, you will be able to publish a wave within a blog and then anyone being able to join this wave from the blog and vice-versa.
Communicating, collaborating on documents, sharing, editing them in real time and playing back "real time" in any type of web clients! Mobile support for Android and iPhone will be available as well.
All this available in open source to increase developers' adherence and engagement around the wave client.
Whatever you like or not, Google Wave, but Yammer, Facebook, twitter, etc, if not replacing your current email, are changing the way we define those old email inboxes.
Let's make sure you surf all waves and get ready for collaboration and innovation!