Forum theatre was a method developed by Augusto Boal in the Latin Americas during the 70ies. It was also called Theatre of the Oppressed. In this process the actors or audience members could stop a performance, often a short scene in which a character was being oppressed in some way. The audience would suggest different actions for the actors to carry out on-stage in an attempt to change the outcome of what they were seeing.
For more info see Wikipedia: http://en.wikipedia.org/wiki/Forum_theatre