
Primarily, Wave is about improving real-time communication on the web. I've been waiting all day for Google to upload the video promised on the Wave homepage, but since it's still not there, here's the skinny from Google's mouth:
What is a wave?
A wave is equal parts conversation and document. People can communicate and work together with richly formatted text, photos, videos, maps, and more.
A wave is shared. Any participant can reply anywhere in the message, edit the content and add participants at any point in the process. Then playback lets anyone rewind the wave to see who said what and when.
A wave is live. With live transmission as you type, participants on a wave can have faster conversations, see edits and interact with extensions in real-time.
A wave is equal parts conversation and document. People can communicate and work together with richly formatted text, photos, videos, maps, and more.
A wave is shared. Any participant can reply anywhere in the message, edit the content and add participants at any point in the process. Then playback lets anyone rewind the wave to see who said what and when.
A wave is live. With live transmission as you type, participants on a wave can have faster conversations, see edits and interact with extensions in real-time.

This addresses an issue that I (and many others much smarter than me) have had with Twitter—namely that it lives on one company's servers, and your information is subject to whatever that company wants to do with it. RSS, email, and IM work using standard protocols that anyone can take advantage of, and since they use standards, I can, for example, send an email from Gmail to someone using Yahoo Mail and know that they'll be able to read it without any problems.
Tech news weblog TechCrunch has a very detailed rundown of Wave (linked below) and the directions Google has taken it so far (it's still incredibly young), but Wave is also very intriguing in the whole future-of-the-web way. It could amount to nothing, but considering the popularity and direction of Twitter and Facebook, the idea of a protocol that turns a similar sort of communication into an open standard sounds very promising.
Wave isn't available in any practical sense to you and me right now, and much of the technology behind Wave requires HTML 5 updates that we won't see completely implemented in most browsers for at least a little while, but you can sign up for Google Wave updates if you're eager to stay updated on any new developments.
Google Wave [via TechCrunch]