The shortest answer is this: an unconference is a highly informal conference. Two differences are particularly notable. First, at an unconference, the program isn’t set beforehand: it’s created on the first day with the help of all the participants rather than beforehand by a program committee. Second, at an unconference, there are no presentations — all participants in an unconference are expected to talk and work with fellow participants <em>in every session</em>. An unconference is to a conference what a seminar is to a lecture; going to an unconference is like being a member of an improv troupe where going to a conference is (mostly) like being a member of an audience. Unconferences are also free or cheap and open to all. For more information, see <a href=”http://en.wikipedia.org/wiki/Unconference” target=”_blank”>Wikipedia’s entry on the unconference</a>.
Some say that the first unconference was BarCamp, which is the model for THATCamp. Read more about BarCamp at <a href=”http://barcamp.org” target=”_blank”>barcamp.org</a>, <a href=”http://radar.oreilly.com/2005/08/bar-camp.html” target=”_blank”>radar.oreilly.com/2005/08/bar-camp.html</a>, and <a href=”http://en.wikipedia.org/wiki/BarCamp” target=”_blank”>en.wikipedia.org/wiki/BarCamp</a>.
