Unconferences are also referred to as 'Open Space Meetings'. They have a number of unusual characteristics, by far the most unusual of which is that there is no preset agenda: it's created by the participants as their first task.
As the participants construct the meeting agenda, you can be sure they get to talk about subjects near and dear to their hearts - stuff they're interested in, as opposed to topics dictated by management. Unconferences are extremely productive when well facilitated and organised around a central, unifying theme.
Here's some more information related to this topic, along with a free guide that will help increase the productivity of your meetings.