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.