An “unconference” is the opposite of a conference, where the agenda is prepared well in advance, with little opportunity for spontaneous collaboration. An unconf is designed by the people who show up, on the day – and participation, rather than passive receptiveness, is expected.

Some further reading:

• http://en.wikibooks.org/wiki/BarCamp_-_How_to_Run_Your_Own
• http://www.unconference.net/unconferencing-how-to-prepare-to-attend-an-unconference/
• http://michaelnielsen.org/blog/how-to-run-an-unconference-some-useful-online-resources/