Waves can consist of any combination conversations (such as email and IM) and documents (collaboration). They provide for rich interaction via text, photos, videos, maps, and more, according to Google. From a usage standpoint, a wave is sort of like an email thread except that it can happen in real time (like IM), is always considered live, and participants can jump in and out of the conversation at any time. A playback capability allows participants to "rewind" the wave at any point and review what's already happened. Edits can be made to any part of the wave at any time, and it's always possible to see who did what. If you think of how an email thread and an IM conversation might be combined into a single entity, that's pretty much a wave.