From their website:
We set out to create a better way to read on-line; our goal was to make something different, engaging, intelligent and digital. Our intent: build an experience that is simultaneously a book group, a computer, and a book.
We believe firmly that people want to read, annotate and discuss, right there, immersed in the text. That’s the best time to talk about a book. We also respect the solitary side to reading: people should have the chance to tune out the community. We wanted it to be attractive, too; to be an experience.
Here is a demo of how it works: