Google Wave is a product coming soon that helps users communicate and collaborate on the web.
A "wave" is equal parts conversation and document, where users can instantly communicate and work together with richly formatted text, photos, videos, maps, and more. The concurrency control technology will let participants reply anywhere in the message, edit the content at the same time and add participants at any point in the process. The program also allows anyone to rewind the wave to see how it has evolved.
Google Wave is also a platform with a rich set of open APIs that allow developers to embed waves in other web services and to build extensions that work inside waves.
Google Wave Overview