Google Wave was announced yesterday and will start to make waves as it goes public in a few months. Google Waves is essentially a real-time communication platform. This article from Mashable is probably the simplest and yet most complete guide at the moment.
It goes into the details of what precisely a Google Wave is plus all the terminology, from wave to wavelet to blip! A Wave is very much like having a global threaded instant messaging history but with the added ability to go back and edit parts of the thread. A Wave can be constructed by a single individual or a group of people, or even by automated robots. Google Wave will be open source so watch out for numerous applications and extensions.