How do you examine this graph?

What do the groups formed in the peripherical areas mean?

What does the central positioning in the graph mean? What are critical nodes and why? What is the interpretation of those items if the graph was representing in the following ways:

(a) Nodes are cities or towns, edges are direct train connections between cities or towns, colours are provinces, and edges are weighted by geographical proximity. (b) Nodes are individuals in a social network, edges are interactions between them (comments, messages) in that social network (weighted by the number of exchanges in a time period), colours are subjects in a list of subjects being examined by the analyst (e.g. weather, health, disasters, elections, etc.) (c) Nodes are pictures, colours are categories of those pictures and edges are weighted by their similarity calculated over a feature space.