An unconference is a free-form event that shares the knowledge-sharing role of a conference but without what many consider to be the negative characteristics of traditional conferences - normally limited to the selective speaker list, the influence of sponsors, the often high cost, and the often-reported idea that the most interesting conversations happen in the hallways rather than in the sessions.
A number of different types of unconference exist. BloggerCon uses pre-defined sessions on a variety of topics, and discussion is then led by a chair, but drawing on attendees. BarCamp allows people to book slots during the day and give either formal presentations or having more informal discussion sessions. The BarCamp format is also used for a wide range of topics (often called "*Camps" or "nichecamps") including developing software for the iPhone ("dev camps", SuperHappyDevCamp, DevHouse, hack days), podcasting (PodCamp), business issues in real estate (REBarCamp), technology in the humanities (ThatCamp) and building a better society in Africa (BarCamp Africa UK).