They have some keynote speakers, but the real driver of attendance is the topic itself and the other attendees. This won't be a 10,000 person mega conference. More likely it'll be a few hundred.
Here's the
Wikipedia page for unconferences. Basically, at the beginning of the conference they ask people who want to run sessions to write their ideas on piece of paper, then during the first session (usually a keynote or something that involves everyone) an organizer lays out the topics onto a big board that acts as the schedule for the rest of the conference (see photo below).
Some people prepare powerpoints or multi-person panels ahead of time with the intention of doing panels. Other people just jot down ideas and have an open discussion.
Unlike big conferences put on by associations, where the main idea is really to make money for the association, unconferences are usually more about the exchange of ideas and networking.
photo from TransportationCamp.org