A community is a network of agents who are all motivated to use a specific resource.
Community is an extremely generic and flexible term here. For example, this term may refer to diverse and overlapping networks which are related through diverse resources. Communities vary immensely in the intensity and frequency of their interactions. However, each community can be identified by the use of a specific resource.
Communities aren't collectives, although they can contain or relate to any number of collectives. Collectives often seek to serve one or more specific communities, with widely varying degrees of success.