1. A system for supporting an arbitrary activity involving collaborators who are not specialists in information technology, the system comprising: a processor which has access to a representation of a model of the activity, the model including model entities, the model entities being in hierarchical relationships one with another, the hierarchical relationships including a first hierarchy and a second hierarchy, the first hierarchy neither containing nor being contained in the second hierarchy, the second hierarchy including a model entity that also belongs to the first hierarchy, and the representation including representations of the model entities and of the hierarchical relationships, the form of the model being defined by the collaborators and the representations of the model entities providing access to information relating to the activity; and an interface to the system for the non-specialist collaborators, the interface being provided by the processor and the interface permitting a collaborator to view and modify the model entities and their hierarchical relationships and to view and modify the information to which the representations of the model entities provide access.