If you're asking what Wave is, you can think of it as collaborative document sharing and commenting (like Google Docs) but with a different document type that supports threaded discussions similar to many online forums.
Wave also had its own email-like inbox for reading updated documents, and was marketed as improved email but it never really worked well for that.
These days I use Google Docs with commenting enabled for the sort of thing we used Wave for.
Not necessarily dissing Wave, but when I first saw it years ago (when it was a Google product) my first thought was "this is like 4chan enterprise!" :)
Wave also had its own email-like inbox for reading updated documents, and was marketed as improved email but it never really worked well for that.
These days I use Google Docs with commenting enabled for the sort of thing we used Wave for.