An unconference is an unstructured gathering or conference, deliberately lacking the structure of conventional conferences. It is participatory and interactive, giving far more opportunity for interaction and freedom to pursue areas of interest.
A common and popular type of unconference is a BarCamp, where participants give talks (typically 20 to 30 minutes), posting them on a schedule board. Specific talks and timings are arranged on the day.
Normally there are at least two or three rooms and participants are encouraged to move to a place where they are either learning or otherwise benefiting, or contributing.
Open Space Technology focuses more on having all participants in the one room, and using techniques to facilitate communication within the group.