The Unconference was borne out of a frustration at larger more formal industry events. Larger events can often be inefficient in their use of time, and not cover topics considered more 'cutting-edge'. Unconference event programmes are usually created by the delegates at the start of the day.
From Wikipedia: 'Unconferences typically feature open discussions rather than having a single speaker at the front of the room giving a talk, although any format is permitted. This form of conference is particularly useful when the attendees generally have a high level of expertise or knowledge in the field the conference convenes to discuss'.