Google Wave is like a combination of forums, email, and IM. You have an inbox that shows different topics, just like your email. But they are stored on a server, like forums are. Unlike forums, you don't see everything, just the stuff you're added to. These things are called "waves". A wave can be added to simultaneously by different people, and it all updates in real time. You can attach documents to a wave, and playback the history of a wave to see what was added, deleted, and changed. You can also discuss in realtime, just like IM.
You can read it about it more
here.