Google Wave is an online communication and collaboration tool that makes real-time interactions more seamless -- in one place, you can communicate and collaborate using richly formatted text, photos, videos, maps, and more.
A wave is a conversation with multiple participants -- participants are people added to a wave to discuss and collaborate on its content. Participants can reply any time and anywhere within a wave, and they can edit content and add more participants as a wave develops. It's also possible to rewind waves with the playback functionality, to see what happened, and when.
In Google Wave you create a wave and add people to it. Everyone on your wave can use richly formatted text, photos, gadgets, and even feeds from other sources on the web. They can insert a reply or edit the wave directly. It's concurrent rich-text editing, where you see on your screen nearly instantly what your fellow collaborators are typing in your wave. That means Google Wave is just as well suited for quick messages as for persistent content — it allows for both collaboration and communication. You can also use "playback" to rewind the wave and see how it evolved.