I don't think Google has been very clear about what Wave really is. There are several components:<p>* The Wave Protocol (<a href="http://www.waveprotocol.org/" rel="nofollow">http://www.waveprotocol.org/</a>) is the heart of the system, and IMO the most interesting part. It's an extension to XMPP, and roughly analogous to SMTP if you're comparing Wave to email. Google will be open sourcing a version of their Wave Protocol server.<p>* The Google Wave client is a GWT application that uses a custom Comet-ish protocol over HTTP to communicate with the server. Analogous to Gmail's web interface or any other email client. I expect this will be open sourced as well, since the Wave Protocol server is useless without a client.<p>* The "robot" API is the JSON-RPC protocol that lets programs interact with Waves to do interesting things like spell checking and translation ("Spelly" and "Rosy" are Google's implementations, likely remaining proprietary). This is separate from the Wave Protocol, but may become a de-facto standard. IMO this is the second most interesting part of Wave. I don't think there's anything quite like this in existing email system (custom server-side spam filters and such?)<p>* The "gadgets" API lets you embed untrusted code to add functionality in Waves.<p>* The "embed" API lets you embed a Wave on a webpage, a la the Google Maps API.<p>Looking at the API docs gives a rough overview of the last 3 (<a href="http://code.google.com/apis/wave/guide.html" rel="nofollow">http://code.google.com/apis/wave/guide.html</a>) and waveprotocol.org covers the first. There's not much info about the GWT client yet.